The NFL Rulebook says :
SECTION 5 - PASS INTERFERENCE
ARTICLE 1. DEFINITION
It is pass interference by either team when any act by a player more than one yard beyond the line of scrimmage significantly hinders an eligible player’s opportunity to catch the ball. Pass interference can only occur when a forward pass is thrown from behind the line of scrimmage, regardless of whether the pass is legal or illegal, or whether it crosses the line.
Defensive pass interference rules apply from the time the ball is thrown until the ball is touched. See Article 2 for prohibited acts while the ball is in the air.
Offensive pass interference rules apply from the time the ball is snapped until the ball is touched. See Article 2 for prohibited acts while the ball is in the air and Article 4 for prohibited acts prior to the pass.
ARTICLE 2. PROHIBITED ACTS BY BOTH TEAMS WHILE THE BALL IS IN THE AIR
Acts that are pass interference include, but are not limited to:
Contact by a player who is not playing the ball that restricts the opponent’s opportunity to make the catch.
Playing through the back of an opponent in an attempt to make a play on the ball.
Grabbing an opponent’s arm(s) in such a manner that restricts his opportunity to catch a pass.
Extending an arm across the body of an opponent, thus restricting his ability to catch a pass, and regardless of whether the player committing such act is playing the ball.
Cutting off the path of an opponent by making contact with him, without playing the ball.
Hooking an opponent in an attempt to get to the ball in such a manner that it causes the opponent’s body to turn prior to the ball arriving.
Initiating contact with an opponent by shoving or pushing off, thus creating a separation in an attempt to catch a pass.
Note: If there is any question whether player contact is incidental, the ruling should be no interference.
ARTICLE 3. PERMISSIBLE ACTS BY BOTH TEAMS WHILE THE BALL IS IN THE AIR
Acts that are permissible by a player include, but are not limited to:
Incidental contact by an opponent’s hands, arms, or body when both players are competing for the ball, or neither player is looking for the ball. If there is any question whether contact is incidental, the ruling shall be no interference.
Inadvertent tangling of feet when both players are playing the ball or neither player is playing the ball.
Contact that would normally be considered pass interference, but the pass is clearly uncatchable by the involved players, except as specified in 8-3-2 and 8-5-4 pertaining to blocking downfield by the offense.
Laying a hand on an opponent that does not restrict him in an attempt to make a play on the ball.
Contact by a player who has gained position on an opponent in an attempt to catch the ball.
Note 1: When the ball is in the air, eligible offensive and defensive receivers have the same right to the path of the ball and are subject to the same restrictions.
Note 2: Acts that do not occur more than one yard beyond the line of scrimmage are not pass interference, but could be offensive or defensive holding (see 12-1-3 and 12-1-6).
Note 3: Whenever a team presents an apparent punting formation and until the ball is kicked, defensive acts that normally constitute pass interference are permitted against the end man on the line of scrimmage, or against an eligible receiver behind the line of scrimmage who is aligned or in motion more than one yard outside the end man on the line, provided that the acts do not constitute illegal holding. Defensive holding, such as tackling a receiver, still can be called and result in a five-yard penalty from the previous spot, if accepted. Offensive pass interference rules still apply.
ARTICLE 4. OTHER PROHIBITED ACTS BY THE OFFENSE
Blocking more than one yard beyond the line of scrimmage by an offensive player prior to a pass being thrown is offensive pass interference. See 8-3-1 for exception for an ineligible offensive player.
Note: It is also pass interference by the offense to block a defender beyond the line while the pass is in the air, if the block occurs in the vicinity of the player to whom the pass is thrown. See 8-3-1-Note for exception for ineligible players.
Penalty: For pass interference by the defense: First down for the offensive team at the spot of the foul. If the interference is also a personal foul (12-2), the 15-yard penalty for such a foul is also enforced, either from the spot of the foul (for interference), or from the end of the run if the foul for pass interference is declined. If the interference is behind the defensive goal line, it is first down for the offensive team on the defense’s one-yard line, or, if the previous spot was inside the two-yard line, halfway between the previous spot and the goal line.
Penalty: For pass interference by the offense: Loss of 10 yards from the previous spot.