Question about unlocked phones

Hello, new to these forums and a former Blackberry user. I am on AT&T and am looking at getting a Bold if it ever comes out. However, pondering a Storm but I cant leave AT&T. I see a lot of references to buying unlocked phones and using them on other networks. How exactly does this work. Do you get full functionality out of them? What do you have to tell your carrier? Just need some basic info on what to do if I buy an unlocked storm (I realize there will be at least 2 makes, 1 which might not be able to be used on AT&T but the other would) and want to use on AT&T....thanks...just noticed posted in wrong forum, sorry.

Unlocked phone is fully functional and you don't have to notify your carrier, just insert your SIM-card.