Unlocked blackberry?

Ok I've been reading a lot on the internet lately about unlocking phones and how cool and cheap it is in the long run. I was wondering I have a blackberry 7290, which my mom got off ebay, yet I was able to put in my sim card (which was originally in a T-Mobile phone) and it worked fine! What does this mean? Is my blackberry unlocked or is it just with T-Mobile like my old phone? If thats the case what will happen if I unlock it? What do I need to do to unlock it?

Thanks a lot very confused!!!