Generally referred to as "pass interference", this can be a penalty against either the offense or the defense. Players are not allowed to make contact with one another further than five yards down the field to interfere with the other player catching the ball.
Offensive pass interference is called when an offensive player pushes the defender away from himself to create separation between the two players in order to get open. This is a 10-yard penalty from the line of scrimmage. Defensive pass interference occurs when a defender makes contact with an offensive player when the ball is in the air, which would prevent him from catching the ball. The penalty for this is assessed at the spot of the foul; if it happens in the end zone, the ball is placed at the one-yard line.