locked means its only compatible with a certain service provider. You can't just stick a Cingular sim card into a T-Mobile phone. I have an un-locked phone, which means with the sim card I got from T Mobile, I can plug that into my un-locked phone( which I bought independently of them) and use it, even though I didn't buy the phone from T-Mobile. There are ways to actually un-lock a locked phone, but I don't know those details.