Questions about buying a cell phone

I’d like to buy a new cell phone. I’ve always gotten them free when I signed a contract, but I still have a year left on my current contract, and I don’t like the phone I have now. So, I’ve been looking on ebay and I think I’ll get one there, but since I’ve never bought a phone on my own and then tried to put it onto my current service and phone number I don’t know how it works.

So, how does it work? Is it really as simple as taking my SIM card out of my current phone and putting it in my new phone? When they say a phone is “unlocked” can I expect it to work w/ my service? Will I need to do any special programming in that case to make it work? Do I need to take it to my service provider and say “make this work” and will there be a fee?

If it matters, I have a Nokia 3595 and I’m looking at the Motorola v180, and my service is w/ AT&T Wireless. While were at it, any opinions about the Motorola v180? It seems to get good reviews.