The catcher crouches directly behind home plate and is primarily responsible for receiving all of a pitcher's pitches.
On defense: 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. Additionally, catchers block pitches thrown into the dirt in front of home plate and "frame" pitches close to the strike zone by slightly moving their catcher's mitts back toward the strike zone after catching pitches.
Catchers play a psychological role, too, taking the occasional trip to the mound to calm a pitcher down or offer advice when he is struggling.
On offense: The catching position is the most physically demanding on the field and arguably requires the most skill of any position on the diamond. As a result, catchers tend to be among the weakest hitters on any given team, and those who can contribute at and behind the plate rank among the game's most valuable assets.