A ROM is generally a reference to a customized version of Android as opposed to what comes stock with the phone (though the carriers actually customize Android to some extent). Bionix, what I'm currently using, is a perfect example. You can read about it here:
[ROM] Bionix1.3 - 3 Flavors [Stock Kernel, Voodoo b4, KingKlick Kernel] *09/20/10* - xda-developers
Yes, it will void your warranty. But if you need to return your phone for some reason you can simply flash back to the stock firmware and remove traces of rooting and running a custom ROM.
I've never had to flash back to stock...yet. It doesn't sound like too much of a chore really.
Yes, you can potentially brick your phone, although most agree the Vibrant is virtually "unbrickable," in that you can almost always recover the phone if something goes awry. ODIN is the program that is used to return the Vibrant back to a factory state.
Yes...if you're rooted you should not be without ROM Manager....
ROM Manager - Android app on AppBrain
ROM Manager (Premium) - Android app on AppBrain
I recommend the paid version...this app is worth it.
Here...do some reading...and there's a lot to read!
[REF]***Vibrant Directory (Bible)***(9/23/2010) - xda-developers
And your questions aren't dumb. You will only be dumb if you fail to ask for help when you need it.