The Definition of the Position
The open end is a specific type of defensive end position in which the player lines up wherever the tight end is not, hence the open side. He is only impeded in his pursuit of the quarterback by the offensive linemen across from him. It is possible that there are multiple open ends on the same play if there is no tight end present or no open ends if there is a tight end on both sides of the offensive line.
The Job of the Open End
The open end is usually one of the best pass rushers on the team because it is easier to rush the passer from the more open side of the field. A mismatch is created as the offensive tackle is left one on one with the open end in his pursuit of the quarterback. Using the best pass rushers available as open ends is a great strategy to use as they will have a great opportunity to sack the quarterback as opposed to lining up across from a tight end too. Offenses must be able to counter open ends by using a running back to help block or find ways to release the ball quickly given the one on one matchup they have with the offensive tackle.