Settlers of Catan is a multiplayer board game designed by Klaus Teuber and first published in 1995. Players assume the roles of settlers, each attempting to build and develop holdings while trading and acquiring resources. Players gain points as their settlements grow; the first to reach a set number of points, typically 10, wins.
Here is a link to the official web site and copies of the official rules for the base game and some official variants. The copies of the rules linked here are my copies that correspond with the versions of the games I own. If you purchased your game recently you should look for more recent rule information from the official website.
These are some good non-official resources and fan created variants of the game.
I have built several versions of the Catan board to play the game on. My largest board had floor tile sized game hexes making for a room sized board to play on. These are the some of the resources I created to make some of my boards.