Flash is easily the most misunderstood class of equipment.  You are faced with a bewildering array of choices.  Let me break it down for you - but the best advice is to avoid using a flash unless absolutely necessary.  Glossary at the bottom.

What type of Flash?

Manual.  A manual flash is one that blasts away at a fixed light level.  Subject brightness will depend on the reflectivity of the scene, the distance from flash to subject, and the aperture of the camera.  All other settings being the same, a white object will appear lighter than a black object.  To obtain the correct exposure, you divide the guide number (for the ISO film or sensitivity) by distance and set the lens aperture to the resulting number.  Voila!  Perfect exposure every time.  Some lenses, such as the 45mm f/2.8 GN Nikkor can automatically adjust the aperture to adjust for distance.  So do 1970s rangefinder cameras with "flashmatic" flash exposure.

Automatic.  An automatic flash incorporates a photocell into the flash that shuts off the flash when the scene reaches a certain brightness (18% grey).  Typically, you set the ISO on the flash, pick a distance range, put the flash on the appropriate automatic setting, set the camera to the indicated aperture, and blast away.  Automatic works well for most scenes, but if your scene is dominated by light or dark objects, you should open up the lens aperture (light scenes) or close it (dark scenes).

TTL.  Essentially, simple TTL flash is automatic flash with the photocell looking through the camera lens.  This works well from a system integration standpoint - modern cameras balance ambient exposure and TTL automatically (with varying degrees of success).  TTL has the advantage in the sense that it doesn't need to know what aperture you are using - it simply looks through the lens and shuts off the flash when there is enough light.  Simple TTL flash often lacks flash exposure compensation and generally looks through the center portion of the frame.  This can be a handicap when you are shooting light or dark scenes and when you need to meter something above the center of the frame.  More sophisticated flashes and bodies add flash exposure compensation and matrix metering (looking to the whole scene and guessing where the subject is).

D-TTL, E-TTL, I-TTL, BS-TTL (collectively, "x-TTL").  Traditional TTL flash takes exposure readings off the film itself as an exposure is made.  The problem with digital sensors (purportedly) is that they don't reflect light the same way as film - and they have no latitude for overexposure.  To address this problem, the charlatans at Nikon and Canon came up with systems to get around this - and destroy the concept of backward compatibility.

The digital "TTL" systems like use a preflash to read the subject's reflectivity - and tailor flash output to match.  On Nikons, the preflash is so subtle that you don't even really notice it.  Put your camera on rear-curtain synch and a long shutter speed and you can see it.  Of course, the conspiracy theorist in my really wonders how the flash measures light when you turn the preflash off.  How?  Probably traditional TTL.

These systems involve little (if any) exposure measurement at the image plane at the time of exposure.  And all of them help your digital camera underexpose flash shots more effectively.

More insidious, though, is the fact that the technology changes from flash to flash.  Fujifilm actually made a digital camera (the Finepix S2) that could use normal TTL flashes.  Nikon started with D-TTL and is now on I-TTL - and I-TTL cameras are not compatible with D-TTL (only) flashes.  D-TTL and I-TTL bodies have zero compatibility with flashes working in traditional TTL mode.  This is puzzling because, as I noted above, the digital bodies have to have some way of measuring light off the image plane when you turn off the preflash.

In reality, none of the digital TTL systems, if shot in low light, produces better results than a $20 automatic flash.  Sorry to be the one to break this to you.

What brand?

Metz.  Metz is the only remaining European manufacturer of flashes.  Their flashes are tough, reliable, solid and expensive.  The Metz 45 automatic flashes, particularly the SCA 300 capable models (45CT-4, 45CL-3, 45CL-4, all GN 148+ in feet) are excellent values on the used market and can be adapted for TTL operation on just about any film system ever made.  The one nice thing about Metzes is that the guide numbers (power ratings) are estimated conservatively.  A small number of newer Metz flashes works with the various x-TTL systems used on digital cameras.  These flashes are fantastically expensive.

Vivitar.  Vivitar makes one flash worth considering: the 283.  This simple, powerful manual and automatic flash (GN 120 in feet) has survived over 30 years of production, first in Japan, then in Korea, then in China.  Every 283 accessory fits every 283 flash ever made.  And there are a lot of accessories from Vivitar and others: power adapters, variable power manual modules, filters, diffusers, bounce attachments, etc.

Sunpak.  Sunpak is a division of Tocad Energy Co (which sounds like it probably also makes nuclear reactors).  It is something of an oddball operation that produces a very low volume of flashes.  Sunpak's forte is a modular TTL system that like the Metz, adapts basic flashes for any number of manufacturers' systems.  Two Sunpak products that stand out are the 120J bare bulb flash (GN160) and the DX-12R ringlight flash.  The former is a super-powerful, relatively diffuse light source.  The latter is the only reasonably-priced ringlight (and the only true ring flash) that can be fitted to a lens with a 77mm filter diameter.  Sunpak's achille's heel is that it has not caught up with the x-TTL digital world.  For things like the 120J that are available with simple automatic modules, this is not a big deal.  For the DX-12R, it leaves you with trial-and-error flash setting when you use a digital camera.

National (Matsushita).  Matsushita (sister company: Panasonic) makes all modern OEM flashes sold by camera manufacturers, whether the nameplate is Nikon, Canon, Pentax, Minolta, or Konica.  You probably already figured this out by comparing a Nikon Speedlight to a similar-spec Canon Speedlite (note the difference in spelling).  These are slick-looking, medium-quality flashes whose power ratings are usually overstated.  If you do extensive manual shooting, you might want to invest in a handheld flash meter.

Batteries for Flash

Alkaline.  Cheap and plentiful.  Very nearly every flash on the market can take alkaline AAs, obtainable very neary every place on the planet.  Rating is usually 1500 MaH (milliamp hours).  Very good storage characteristics and low self-discharge.

A little known fact about alkalines is that when they leak (often when run completely out of energy), the "acid" can be removed with white vinegar and a Q-tip (cotton swab).  Make sure all parts of the flash are completely dry before powering it up again.

Lithium AA.  These actually run at 1.8V and have huge power ratings (3600 MaH).  They are sold only by Everready.  These are good for high-drain applications, like shooting a ton of flash shots all at once.  Not rechargeable, and if you are simply going to put these in a flash, use them once, and basically leave them to rot, you might as well use conventional alkaline AAs, which have similar (if not better) storage characteristics.  Lithium AAs do store very well in the package, though, until you actually start using them.

Lithim-Ion (Li-Ion). These are only beginning to come into fashion for flashes.  Fast charging, high energy per unit weight, and short service life (typically about 400 cycles).  Reach well to shallow discharges.  This is the type of cell found in a mobile telephone.  Not yet available in AA, but we can probably expect to see them soon.  Bad self-discharge - can be as much as 3% per day.

Nickel-Cadmium (NiCd).  These rechargeable batteries run at 1.2V instead of 1.5V.  Your flash will never know the difference.  These are the most primitive form of rechargeable AA battery.  They are also still the best choice if you drain the hell out of your batteries every time you use them.  They do not like to be half-discharged and then recharged.  You must dispose of these batteries as hazardous waste.  Rating is usually around 900-1000 MaH.  Good storage characteristics.

Nickel Metal Hydride (NiMH).  These rechargeables have less of a memory effect than NiCd and run at 1.2-1.4V.  They can also be used in chargers that can fill up a set of AAs in 15 minutes.  Their lifespan is directly proportional to how long it takes to charge them.  Use them in a 5-hour charger, and you may get 1000 recharges.  In a 15-minute charger, you may get a lot less.  These are heavier than alkalines.  Power ratings range from 1000 to 2500 MaH (and almost always overstated).  These drain very quickly on their own when left in device.

Sealed lead acid.  You usually see these as Quantum batteries.  Huge power capacity, good storage life, good for a lot of recharges.  But heavy and expensive.

Glossary - electronic flash

"Bare bulb" flash.  These are tube flashes where the Xenon tube is curled around in a glass housing that resembles a light bulb.  These are powerful and popular for weddings and portraiture.  Pointed at a white ceiling, these can simulate a powerful incandescent room light.

Dedicated flash.  This is a type of flash that incorporates camera-specific functions.  Dedicated flashes can trigger a ready light in the viewfinder, set camera aperture, receive shooting data from the camera, and do any number of neat things. TTL operation is a variety of dedlcted flash operation.  A dedicated flash shoe on a camera typically has one contact in the center (the flash trigger) and other contacts that accommodate other functions.

Guide number.  This is a concept left over from bulb flash days.  Because flash bulbs only really have one setting (burn, baby burn), the only way to control output is lens aperture (and to some limited degree, shutter speed - but we can ignore that with electronic flash).  You would divide guide number by distance (indicated by a scale on your Speed Graphic after you focused) to get the shooting aperture.  Guide numbers increase with the sensitivity of your film or sensor - so each flash has a guide number for each ISO rating.

Hot shoe.  This is an ISO shoe with a contact in the center.  When the camera fires, it completes a circuit that runs from the metal edge of the shoe to a contact on the flash, through the flash, through the center contact on the flash foot and into the contact in the middle of the camera's accessory shoe.  In other words, you don't need the PC cord.

ISO ("cold") shoe. This is a standardized accessory shoe found on most 35mm cameras, most digital cameras, and some medium format cameras.  It is designed to accept a flash, an accessory light meter, a remote flash trigger, a level, and any other number of accessories.  Flashes mounted in an ISO shoe need a cord to connect them to the camera body.

"Leaf shutter lens."  In the olden days, many cameras used leaf shutters.  These opened like flower petals and could be used with flash at any shutter speed.  After a while, focal plane shutters became popular.  These, however, were generally limited to using flash at 1/30 sec or slower (later 1/60, 1/90, 1/125, and 1/250 sec). 

The leaf shutter lens was developed for cameras like the Pentax 6x7 (synch speed 1/30 sec), so that you could use fill flash in daylight.  The Pentax 6x7 90mm leaf shutter lens has its own shutter (1/60-1/250 sec) that is manually cocked and triggered by the camera body.  Compur made a similar shutter for a leaf shutter lens for Leica bodies (synch speed 1/50 sec) in the 1950s.

If you can do so, just buy a camera whose lenses incorporate leaf shutters by design, rather than going the retrofit "leaf shutter lens" route.

PC cord.  This is generally a small, friction-fit flash cord.  Most cameras have PC terminals.  More sophisticated cameras add threading to the PC terminals so you can use locking cords that won't fall out.  The PC connection replaced the earlier ASA bayonet connector, which you sometimes see looking like a pointy metal nipple on older leaf shutters.

Ring flash.  Used for macro or fashion work.   Same as above, but the tube curves around in a housing concentric with the camera lens.  Newer "ring flashes" like the Nikon SB-29 are just two bulbs, one on each side of the lens.

Thyristor.  Born in the energy crisis, this device saves battery power.  On automatic and TTL flashes, the flash is usually shut off before it completely discharges the capacitors (which store the huge amount of electricity required to shoot off the Xenon tube).  A thyristor diverts any extra energy back into the capacitors.  A non-thyristor automatic flash will use the same amount of energy to fire for 1/20,000 sec as it does for 1/200 sec.  Accordingly, the batteries will run out faster on a non-thyristor unit.  Most flashes are now thyristor-driven (the notable exception being small, 2xAA flashes).

Trigger voltage.  In every system, camera electronics make up part of the flash circuit.  Trigger voltage is a measure of how much electricity is going through your camera every time you take a flash shot. Modern cameras (except for some like the pro Nikons) need to keep this quite low - unless you use something like a Wein Safe-Sync to keep the excess electricity out of your camera body.  You can measure flash trigger voltage with a $20 multimeter from Radio Shack.

Tube flash.  This is a flash with a straight-line Xenon gas tube.  The tube fires with a jolt of high-voltage electricity.

Glossary - older flash

Bulb flash.  Imagine a light bulb - but instead of a durable filament in argon gas, it contains highly flammable magnesium foil in pure oxygen. This was the brutal reality of flash photography until the late 1960s - but better than having your face burned off by magnesium flash powder. 

Bulb flashes are one-shot affairs and the bulbs themselves came in varying sizes (read: power) and color balances (usually daylight).

The original Star Wars lightsaber was a Heiland Speed Graphic flash handle.

M bulbs/M synch.  M bulbs are flash bulbs with slow ignition and quick burn.  Typically, a shutter set to M synch will start the flash process 20ms before the shutter opens completely.

FP bulbs.  These have a slower burn time, to accommodate focal plane shutters - which expose the frame by moving a slit of light across the film.  These are for use with the Speed Graphic's focal plane shutter or SLR cameras that have an M terminal, like the Konica Autoreflex T.

X synch.  X-synch fires the flash exactly at the moment when the shutter is all the way open.   It can be used at any speed on a leaf shutter, but it can only be used at (or below) the maximum synch speed of a camera with a focal plane shutter.  No one talks about this anymore - all modern cameras have X-synch only.