You know how in baseball you can only score runs in front of you? Well in cricket the pitch is in the center of the ground not at the end, so you can score runs in front and behind.
In baseball if you miss the ball, the dude behind you wearing the mask catches it. If he catches it thrice you are out.
In cricket the guy behind you is called the wicket keeper. If it is a slow bowler who turns the ball with his wrist and fingers he stands up close and if it is fast bowler (bowlers are the equivalent of pitchers) who runs in and tries to take your head off then the wicket keeper stands much further back.
He is called a wicket keeper because between you and the wicket keeper there are three sticks (called stumps or wickets) with bails on them. If the ball hits the wicket dislodging the bails you are out. But that's why he is called a wicket keeper.
