While I agree with ACD that you should do a little searching, the unrevoked site is not user friendly at all. Basically:
unrevoked = one time root. basically temporarily "unlocks" your phone to allow you to install a custom recovery.
unrevoked forever = nand unlock. This is a permanent "unlock" which allows you to install unsigned firmware. So even if you accept a locked update, you can still install a custom recovery.
Before unrevoked 3.2, if you were to install the 2.2 OTA, you would not be able to root without having previously done unrevoked forever.