The catcher crouches directly behind home plate and is primarily responsible for receiving all of a pitcher's pitches. Catchers have many duties on defense. Primarily, they are responsible for catching all pitches thrown by their teams' pitchers, often suggesting pitch selection by way of pre-pitch hand signals displayed between their legs and out of sight from the opposing team. Catchers are also asked to throw to second base and third base in order to prevent runners from stealing bags.

Source: Major League Baseball Official Site