I'll try.

First, some basic concepts - Light from a point souce (e.g., your flash) gets dimmer the farther away it is from the source, this is the reason you can see in the dark with a flashlight out to 10 or 15 feet, but shine it 100 feet away and it's pretty 'weak'; shine it at something in your hands and your eyes hurt as they try to adjust. The light fall-off obeys something called "the inverse square law"; this says something like "light intensity is proportional to the inverse of the square of the distance from the source". In photography, we use "f-stops" as one measure of light intensity. For a flash unit, we've developed a standard we call 'Guide Number' (GN) that is an expression of the power of the flash; it's simply the f-stop times the effective distance of the flash for a given ISO and coverage. As an example, for an ISO of 100, a typical full-power shoe-mounted flash has a GN of 110 for a 35mm lens coverage, expressed in feet. This would give some thoretical maximimum exposures / distances:

• f/2 at 55 feet
• f/2.8 at 39.3 feet
• f/4 at 27.5 feet
• f/5.6 at 19.6 feet
• f/8 at 13.75 feet
• f/11 at 10 feet
• f/16 at 6.9 feet
• f/22 at 5 feet

This defines 'the physics' of flashes, and why those of us who understand it laugh at others who use P&S flash cameras whale watching, at major sporting events from hundreds of feet away, and at the rim of the Grand Canyon.

Features that are desirable on most flashes, regardless of 'dedication':

• Auto mode - the ability for the flash to sense when 'enough light' (according to the information it has been given) has fallen on the subject to give a 'correct exposure'. This is handy because it allows you to use the flash in broadly defined camera-to-subject 'ranges' (e.g. '5 to 39 feet') without resetting your exposure. Auto mode relies on the flash's own 'electric eye' sensor to control the exposure.
• Manual mode - sometimes 'Auto' gets fooled; the white wedding dress, the black tux, etc. For similar reasons that you would want 'manual control' of the exposure on your camera, manual mode on a flash is handy sometimes.
• Bounce flash - direct on-camera flash has a 'look' that's pretty unnatural, kinda like shining a flashlight in somebody's face. Also, it can produce red-eye. Bounce flash allows you to tilt the flash head up to bounce it off of a white (or light neutral color) ceiling. When the flash returns from the ceiling to light your subject, the 'character' of the light has changed in two important ways: one, it comes from above, which is the way we have come to expect lighting to 'naturally' appear; and two, it is much softer in appearance. It's 'softer' because the light source is now 'larger'. Instead of the light coming from the 4 square inches of the flash head, it now comes from the 10 square feet of the illuminated portion of the ceiling. This is like the difference between harsh sunlight and the soft light of an overcast day. Here's an example of bounce flash:
  http://www.thepeaches.com/photography/MattKatie.htm
  Sorry, I didn't shoot the same thing without bounce for comparison.
• Swivel head - same thing as bounce flash, but allows you to bounce with an on-camera flash when it's held vertically for portraits
• Bounce Card or Second Flash Tube - Bounce can create it's own shadows, most objectionably in the eye socket area. A bounce card is simply a white surface that pulls out from the flash head to kick a little light straight forward to fill in shadows (used in linked photos above) when the flash head is pointed up at the ceiling. This is a feature that emulates all the photojournalists who used a simple white recipe card taped / rubber banded to the head of their Vivitar 283's for decades. The second flash tube accomplishes the same thing, kicking a little light forward when bouncing (I have no experience with a two tube system)
• Energy saving circuit, aka 'Thyristor' - we're probably way past this, but I'll throw it in. An auto flash unit works by 'cutting off' the flash pulse when enough light has hit the target. A full power flash pulse will be something about 1/1000th of a second in duration, a minimum power flash pulse might be down around 1/32,000th of a second. It will take most shoe-mounted flash units anywhere from five seconds to twenty-five seconds (depending on how good your batteries are) to recover from a full power discharge. In the early days of 'Auto Flash', these 'recycle times' were common even if your 'Auto' function cut the flash off early. The 'saved power' was simply wasted. A thyristor energy saving circuit re-routed this saved power back into the charge circuit, enabling vastly reduced recycling times. If your 'Auto Range' is 5 to 39 feet, you'll get very fast recycling at five feet, and very slow recycling at 39 feet. 'Fast Recycling' means you can bang off 2, 3, 4, or 5 frames (watching for reactions) in the space of 5 to 10 seconds.
• AA batteries - In my part of the world, these things are cheap and available everywhere. In my opinion, these should be the standard for power of an on-camera flash, usually 4 of them. Little lithiums (like the something-123 size) are very powerful for their size, but when they go dead in the middle of a birthday party, it's pretty unlikely that anybody in the house will have spares; it's also less likely that the corner convenience store will have them
• External power - if you get serious about this stuff, you'll want to investigate a separate external (i.e, clipped to your belt) battery. These have more capacity than the batteries in the flash. They usually give shorter recycle times, and allow more flashes per 'charge'. They sometimes connect to the flash through a separate connector, and sometimes they have a 'fake battery' shaped device that plugs in where the batteries go. FWIW, I've been shooting 'seriously' for 25 years (some for money, mostly for hobby) and I've never felt the need for external power.
• Remote auto sensor - Another way to get away from the odd look of direct on-camera flash is to move the flash away from the camera. If you do that, it's helpful if you can leave the flash's auto sensor on the camera, pointed at the subject. Vivitar 283 (and many others, but this is the flash I have 'many miles' on) allows you to unplug the sensor, leave it on the camera, and connect it via a special cord to the flash. 'Quick and dirty', you can hold the camera in your right hand, flash held in your left high overhead, and get a better 'look' than having the flash on camera.
• Zoom head - A flash with a fixed head will typically cover the angle seen by a 35mm lens. A zoom head on the flash allows the flash to vary the angle of the light output; a typical 'zoom range' is 24mm to 85mm. If you shoot with lenses wider than 35mm and the flash isn't 'wide enough' you'll see dark corners in your photos. At the other end, the reason to 'narrow' the flash beam down (to the 85mm setting) is to increase the flash's effective range when working with telephoto lenses. Some flashes accomplish the same thing as a zoom head by using accessory wide / angle and telephoto 'filters' on the flash; my Vivitar 283 did this.
• Slave Flash - A slave flash fires whenever it senses another flash in the room firing. The slave circuit is simply a Light Activated Silicon Controlled Rectifier (LASCR). You can pick up a LASCR slave for about $15 and turn any flash into a slave. Using several slave flashes can help light large areas or difficult subjects. In general, using multiple flashes means they're all set on 'manual' as the light from one fools the auto settings of the other.

Next, a discussion of 'dedicated' flash features for Nikon cameras. The original flash 'hot shoe' standard for 35mm cameras standardized the size, and the location of the central 'trigger' contact (the ground or return is in the hot shoe 'rail'. As each camera manufacturer added features to 'their' flash systems, they added additional contacts. If you stick with the flashes marketed by the camera manufacturer, you generally get the advanced features working properly. Many '3rd party' flash manufacturers like Vivitar, Sunpak and Metz use 'dedication modules' to enable the specific features for a given camera. I refer to all of the above as 'dedicated flashes'.

• Ready light indication in the viewfinder - In Nikon, this was the very first 'dedicated feature' (if you ignore the weird-ass non-standard flash shoes). Simply, you don't have to look up at the flash to see if it's fully charged and ready to fire, there's some indication in the viewfinder.
• Auto setting of camera sync speed - in general flashes only work at or below a given camera's 'sync speed'. Due to the way that a typical 35mm SLR's shutter works, there is an upper limit to the speed that a camera flash can work. Above this speed, the shutter curtains form a 'travelling slit', and your photos will have a significant black portion, essentially a 'shadow' of one of the curtains. Auto setting of camera sync speed is generally a blessing for the novice (it keeps them from making a really 'fatal' mistake) and a bit of a curse for the advanced amateur / pro. Early versions of this 'feature' locked one in to 'the' sync speed, and didn't allow anything to be set slower. Slower shutter speeds still work with the flash, but they allow more ambient light to register on the film; this gives a more natural look of mixed flash / available light. Later versions of this feature typically allow some flexibility to select slower speeds, but you may have to set your camera to more advanced 'modes' (i.e., not 'program') to get there.
• Through The Lens (TTL) flash metering - This is where things really got interesting. TTL is a form of 'Auto Mode' where the flash sensor is in the camera body, usually down in the mirror box looking at the film. TTL solved a number of problems:
• Variable aperture zoom lenses - these lenses change effective 'speed' when zoomed from the short end to the long end (like my Nikon 24-50mm f/3.3-4.5 AFD). Without TTL flash metering, you would have to account for this speed change when zooming to get a really correct flash exposure
• Filter factors - TTL 'knows' you are shooting through a filter that cuts down on your light, like a polarizer
• Lens extension - If you're shooting with a 'true macro' lens (one that gets you down to half life size or closer) your lens is no longer as 'fast' as you may think it is (i.e., the Nikon 105 f/2.8 AFD Micro is really something like f/5 or f/5.6 at it's closest focus point); TTL can 'see' this and compensate
• Ignores obstructions that aren't visible to the lens - Imagine that you're in the crush of folks that are trying to get a glimpse of someone famous (pope, president, you name it). you have a 200mm lens on your camera, but your flash covers a wider angle, say 35mm. There are people in front of you, but your 200mm lens lets you get a 'clear shot' in between their shoulders of your subject. In the 'bad old days' (before TTL), your old 'Auto Flash' would 'see' the shoulders closer in, and the flash would expose for them, rendering your subject grossly underexposed. TTL ignores anything it can't see through the lens, and gives a good exposure in that situation.
• Slow Sync - see also 'Auto setting of camera sync speed' above. Slow sync allows the camera to set a sync speed slower than the typical fast shutter speed. Typically, it allows the sync speed to go well down into the range where it's not adisable to handhold the camera. Slow sync allows the ambient light to 'register' on your exposure, and can give a more natural looking combination of lighting. Setting of 'slow sync' varies in the Nikon system depending on the specific camera and flash (sometimes it's on the flash, sometimes it's on the camera, sometimes you can't get there from here). Slow sync uses the TTL exposure system.
• Rear Curtain Sync - this is a specific 'flavor' of slow sync, where the flash is fired just before the camera 'closes', not just as the camera 'opens'. This is most dramatic if the shutter speed is slower than 1/30th of a second, and if there is motion involved. Sample scenario: you want to photograph a cyclist at night, and he has lights and one of those 'don't hit me' strobes on his bike. You set up your camera on a tripod and have him ride past you as you shoot. Your camera is set for one full second exposure. With ordinary 'slow sync', you'll get a 'frozen' flash image, with the bike lights and strobes painting trails in front of the flash image. With rear curtain sync, the bike lights and strobes will paint trails behind the frozen flash image, which most people feel conveys motion better. A buddy of mine uses this for a simpler 'psychological' reason: he shoots weddings, and he'll often do a shot at a reception with the happy couple in front of the city skyline, maybe a five or ten second exposure. It's easier to do this with 'rear sync', since we are conditioned to think 'when the flash goes off, we're done, I can move off now'. A camera has to have 'rear sync' designed in as a feature, you can't take just any old Nikon camera, slap a 'rear sync capable' flash on it and expect it to work. Rear sync uses the TTL exposure system.
• High Speed FP sync - this is a special mode that allows you to shoot flash above the normal sync speed. The flash will put out a rapidly pulsing flash (think 'disco light') during the exposure. The theory is that the light will be essentially 'continuous' while the travelling shutter slit moves. This solves problems for a small number of people, but in practice, it's pretty hard to use. It's a little like using a low-powered manual flash unit, and you need to have a great deal of flexibility in your setup to work around the limitations. High Speed FP sync does not use the TTL exposure system.
• Matrix Balanced Fill Flash (MBFF) - This is Nikon getting really fancy, trying to match the flash light to the ambient light. This system 'assumes' that there is some ambient light, and that the flash is provding 'fill' to fill in the shadows. In order to make the flash a bit more 'subtle', it's set up to underexpose a touch, I think it's about 0.7 stops of underexposure. In general, the system works very well, and provides the best possible flash exposures with minimal 'thought' from the photographer. MBFF uses the TTL exposure system. It has a few 'quirks':
  • Where the flash is the main light source (total darkness or low ambient light levels), it can give underexposure (by that 0.7 stops mentioned above). Nikon advises disabling (MBFF) in these conditions. With color print film, most people won't notice anyway.
  • In Program mode, Nikon 'locks out' fast apertures as a function of the ISO. At ISO 200, you can't select an aperture faster than f/4, at ISO 100, no faster than f/2.8, etc. (correct me if I'm wrong here, I'm working from memory). This is to prevent 'burnout' if you're close to your subject; the flash can only 'cut off' so fast, and if you're shooting wide open at 4 feet, you're going to get overexposure.
  • The system will set shutter speeds between 1/60th and max sync (1/250th on most of my Nikon AF cameras) to try to allow for ambient light capture. A perennial question to www.photo.net goes something like, "Why does my N90s set the shutter speed to 1/60th when I use the flash? I thought this camera synced at 1/250th?" The answer is most people only use the flash when there's not enough light to make an exposure without it. In that instance, a shutter speed of 1/60th lets the ambient light 'register' in the photo. Setting the camera or flash ('how' varies from model to model) to 'slow sync' or 'rear sync' lets the camera 'break out' of the 1/60th lower limit (some cameras also have 'custom functions' to alter this threshold).
• 3D Matrix Balanced Fill Flash (3DMBFF) - Everything I said about MBFF above, plus the 'D' (distance) information from Nikon's 'D' series of lenses. 'D' lenses (on Nikon AF cameras made after the N90/F90, where the feature was introduced) report the focus distance back to the camera. This can be used in the flash calculation (a side note, it's used in the non-flash exposure calculation as well, but most of us who 'think we know' how this stuff works can't name a use for it without flash). 3DMBFF uses the TTL exposure system. The specific quirks of 3DMBFF:
  • 3DMBFF has the ability to 'factor in' differences in subject reflectance into the flash exposure equation, it does this by firing a 'pre-flash' and evaluating it. Let's assume a 'white wedding dress' as our main subject. When you push the shutter release, the camera fires a pre-flash. If your camera is focussed at 10 feet, the system knows this. It evaluates the light returning from the pre-flash, compares it to the 'expected' (i.e., the what you'd get from an 18% gray subject at 10 feet) amount, and determines, 'A-ha! This is a very bright subject!'; and it biases in some overexposure (perhaps +1 stop) into the following flash exposure
  • 3DMBFF thinks it 'knows' the distance from the flash to the subject. It can be fooled by 'light modifiers' (diffusers, softboxes) added to the light path
  • 3DMBFF doesn't work in bounce mode. If you take a Nikon flash out of its 'direct' mode (where it's pointed straight ahead), 3DMBFF is automatically disabled
  • The pre-flash screws up most flash meters and slave flash units. They will trigger on the 'pre-flash' and either not read correctly or not fire at the right time. This is of little concern to the vast majority of non-pro users of the systems
• Auto Zoom - The camera will tell the flash what focal length the lens is set for (for lenses with a 'chip' that communicates with the camera, generally the AF lenses) and the flash will respond by setting the most appropriate focal length on the zoom head of the flash. This ensures the maximum possible efficiency of the flash while maintaining the appropriate coverage.
• Remote Sensing - similar to what I said above in the 'non-dedicated' portion. In Nikon, the 'basic building block' for remote sensing is the hideously expensive SC-17 remote cord (about $60 from B&H). This cord allows full-TTL coupling between Nikon-dedicated flashes and cameras. Note that with the SC-17 it is possible to point the flash in non-'direct path' methods that can 'spoof' the 3DMBFF system (see notes on that above). It's also possible to use the flash with the SB-17 in a 'direct path' fashion with the flash folded out flat that unfortunately disables 3DMBFF.

As you can probably guess, the subject of Nikon flashes and how they interact with various Nikon cameras can 'fill a book'. It does; I recommend for 'advanced study' Thom Hogan's book on the subject, you can find it at www.bythom.com.

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