I like to think of role playing games as group storytelling. There are many ways to go about doing this, but it is most simply described as this: A group of friends gather and one of them thinks up a story, complete with characters and an intended plot. From here, the person who thought up the story (called the Game Master (GM)) allows each of the other people select a character in his or her story. Once another person chooses a character, they become a Player in the GM's story, and the GM can no longer control that character. All these players, under the direction of the GM, try to tell the story as they see it from their characters' points of view. This is usually where players run amok and the GM's story turns out to be entirely different from what he or she intended, but that's the fun--It's like unfolding a mystery. :)

This is only one of many theories and one of many ways to go about it. If you visit the links portion of my webpage, you can find some other sites that talk about or involve role playing and RPGs. ;)