Unlocked Cell Phones?

Okay, I know that I need to get a new cell phone before I head off on my ultimate road trip, since I want one of those ones I can use to send text messages that slides open to reveal a keyboard hidden away inside the body of the phone, but I am not exactly the most phone savy person. What does it mean when someone says unlocked cell phones? Are those phones that work with any phone company, so you can buy it and have your own phone company activate it for you? Or does unlocked mean something else? Any input that anyone has on this would be appreciated. Thanks.