Well, I don't know if you've already seen the article about LUA on our frontpage. It deals with integrating LUA into a game so it may be a bit technical, but it should give you a pretty good idea of what you can do with LUA and it contains some interesting links.

Basically LUA is limited by what functionality the game developer chooses to expose through it, so if you are looking for info about using LUA for AI for example, you'd probably find the more info on the site of the game your scripting for. I don't think LUA is really used for level design though, but if it is, I'd be interested to read more about it :)