There's a good discussion of this on wikipedia. There's basically three variables: disc hub mount, disc diameter, and caliper mount. The hub and the disc must match so you can bolt the disc to the hub and there are basically two standards - center-loc used by Shimano and an IS six-bolt used by most everybody else. The caliper must be designed for a certain disc diameter (e.g., 160mm). Finally, the caliper must have the proper mounting holes to bolt to the fork leg and there are basically two standards here - IS and post mount, although there is a simple adapter to convert from one to the other.
If you buy a common disc brake kit it will come with matched up caliper and rotor, and probably will have the IS to post-mount adapter, so you just need to make sure you get the right hub. Unless you're going Shimano, you probably will want a six-bolt IS hub.
Hyd vs. mechanical gets into what you have to bolt up on the handlebar - obviously, you have to have a different brake lever to have the master cylinder. If you're doing a retrofit, you probably just want to stick to mechanical and then you can use your existing brake levers.
Avid BB5's and BB7's are very well liked for a mechanical disc brake.
All this from someone who has spec'd some brakes out, but never actually done a retrofit. So keep in mind what you paid for this advice.