Unlocked RAZR V3?

I had a Sony Ericsson z520a that croaked (long story), and I found an unlocked RAZR on Amazon for 120 bucks, but how exactly does this unlocked stuff work? Can I just put my SIM card in, and I'll have the same plan (unlimited texts, etc.), and everything will be the same? Or can I not do anything with an unlocked phone but call? I have AT&T by the way.