In American football, a bootleg is a play in which the quarterback fakes a handoff to a running back and then moves out of the pocket in a curving motion toward one of the two sidelines.
Once the quarterback is outside the pocket, he can either pass the ball, or run with it. In some cases the quarterback moves in the direction of his linemen and thus has blockers in front of him. In other cases, he moves away from his own blockers, which is known as a "naked" bootleg.
The bootleg is deployed in hopes of serving a variety of purposes. First and foremost, it is intended to confuse the defense as to where the ball actually is. The defense may think the running back actually received the ball, buying time for the quarterback to survey the field for a receiver to pass to, or run the ball himself. The bootleg also confuses the defense as to whether the quarterback intends to pass or run, often "freezing" the defense and opening up running lanes and passing opportunities. Especially in the case of a naked bootleg, the quarterback may attract the attention of linebackers or defensive backs, allowing receivers to come open. The bootleg also gets the quarterback out of the pocket, so if nobody is open and he has to throw the ball away to avoid a sack, he won't be penalized for intentional grounding. In addition, the bootleg also takes a while to develop, so it allows receivers more time to get farther downfield if the quarterback is looking to throw a long bomb.
Bootlegs are usually run with mobile quarterbacks who are threats to both pass or run. However, once in a while, a less mobile quarterback will run a bootleg in an attempt to catch the defense off guard.
The name "bootleg" was given to this play by analogy to bootleggers during Prohibition, as the quarterback, after faking the handoff, often hides the ball behind his hip, similar to the way Prohibition bootleggers hid a bottle of alcohol.
If the quarterback does not fake a handoff and simply moves toward the sideline right away, it is usually not called a "bootleg" but rather a "rollout."