It is not anything like Monopoly or Trivial Pursuit.
You don't act out the character really, you just determine what the character would do or say.
The best way to describe gaming is this:
A group of friends get together on a Saturday night to play. They all sit around a table and one person sits at the end, that person is the GM or gamemaster. Their main function is to tell the story. The other players play characters (depending on the game it can be superheroes or Pirates) and their decisions affect the story. When they try to affect the story, say by trying to parley with an enemy pirate, they would roll a dice, usually a twenty sided one, and if they get a certian number or above then they succeed, if they don't they fail horribly and something goes wrong. Then the characters will respond to that as well.
Usually you create your own character, but when you go to conventions, you get to play a pregenerated character that was created by the GM.
I hope that helped a little bit.
There is a really great book out there if you are looking for an interesting read, it won the ENnie this year and was really well deserved. It is called
Confessions of a Part Time Sorceress: A Girls Guide to D&D by Shelley Mazzanoble. It not only is a great book, but it has some great receipes to try!