If you're not familiar with the Settlers of Catan board game, the level of involvement Catan requires maybe a little off-putting at first. There are tiles with pictures, numbers and dots; several different types of game pieces; two different sets of cards; and dice all being used at once throughout the game. It may appear complex (and even chaotic) at first, but learning the basic gameplay proves to be surprisingly easy.
The game board is made up of hexagonal tiles, each representing one of five resources: lumber, wool, brick, grain, and ore. Each player begins with two roads and two settlements to place anywhere on the board as a starting point. Each tile also has a number, and any time that number is rolled on the dice, any settlements or cities bordering that tile receive a corresponding resource card, which can be used to buy more cities and so forth. Players receive victory points for building each additional settlement and city, and the first player to accumulate ten victory points wins.
That's the basic idea of the gameplay, but there are many other strategic factors involved, like bartering with other players for resources to make mutually beneficial deals, as well as using "development cards" to perform special actions, like stealing from other players or temporarily monopolizing a resource.