Hacking Action Replay Cheat Codes

[iwakura]

Sorry if this is the wrong place to put this thread, but I couldn't find anywhere else.
I've never used the cheat functions on flashcarts, and I never plan to. However, as a reviewer, I figure I should really familiarize myself with these, since other people seem to love cheating.

This leads me to a few questions:
1. what types of cheats are there? I've heard of this "action replay", but what else is there?
2. are user-added cheat codes the same as action replay?
3. do cheats/cheating functions differ from cart to cart for action replay cheats? for user-added cheats?

thanks!