A penalty is an infraction of the rules. In American football, the specific penalties and their ramifications can get pretty complicated. These penalties cost the offending football team 10 yards:
Deliberately batting or punching a loose ball: When a player bats or punches a loose ball toward an opponent’s goal line, or in any direction if the loose ball is in either end zone. An offensive player can’t bat forward a ball in a player’s possession or a backward pass in flight.
Deliberately kicking a loose ball: When a player kicks a loose ball in the field of play or tries to kick a ball from a player’s possession. The ball isn’t dead when an illegal kick is recovered, however.
Helping the runner: When a member of the offensive team pushes or pulls a runner forward when the defense has already stopped his momentum.
Holding, illegal use of the hands, arms, or body by the offense: When an offensive player uses his hands, arms, or other parts of his body to hold a defensive player from tackling the ball carrier. The penalty is most common when linemen are attempting to protect the quarterback from being sacked (tackled behind the line of scrimmage).
The defense is also guilty of holding when it tackles or prevents an offensive player, other than the ball carrier, from moving downfield after the ball is snapped. On a punt, field goal attempt, or extra point try, the defense can’t grab or pull an offensive blocker in order to clear a path for a teammate to block the kick or punt attempt.
Offensive pass interference: When a forward pass is thrown and an offensive player physically restricts or impedes a defender in a manner that’s visually evident and materially affects the opponent’s opportunity to gain position to catch the ball. This penalty usually occurs when a pass is badly underthrown and the intended receiver must come back to the ball and interfere rather than allow the defender to intercept the pass.
Tripping a member of either team: When a player, usually close to the line of scrimmage, sees someone running past him and sticks out his leg or foot, tripping the player.