One of the most common penalties to get called in the NFL is a holding penalty. A flag will get thrown for holding if a player is illegally restraining another player who is not in possession of the football. Most of the time you will see a flag get thrown for holding at least once in a game, but usually much more than that. It is very common for players on both sides of the ball to get called for this. Typically, a flag for holding will get thrown if a player grabs the jersey of another player not holding the ball and throws him to the ground when he is not holding the ball. This penalty is mostly called on the offense when a run blocker grabs or throws a defender to the ground to open up space for the running back, or when a pass blocker grabs onto the defender to prevent him from getting to the quarterback. This penalty is mostly called on the defense if the defender grabs onto the player opposite him outside of the 5-yard area from the line of scrimmage where he is legally allowed to touch an offensive player. If the defender touches the opponent outside of the 5-yard area, he will get the penalty called against him.
If a holding penalty is called on the defense, it is a 5 yard penalty and an automatic first down for the offense. If the holding penalty is called on the offense, it is a 10 yard penalty and whichever down it is gets replayed.