A pull-up is a variety of upper-body compound pulling exercises. The pull-up has two main grips: palms facing forward/away, and palms facing the person.
The most popular current meaning refers to a closed-chain bodyweight movement where the body is suspended by the arms, gripping something, and pulls up. As this happens, the wrists remain in neutral (straight, neither flexed nor extended) position, the elbows flex and the shoulder adducts and/or extends to bring the elbows to or sometimes behind the torso. The knees may be bent by choice or if the bar is not high enough. Bending the knees may reduce pendulum-type swinging.
A traditional pull-up relies on upper body strength with no swinging or "kipping" using a forceful initial movement of the legs in order to gain momentum). The exercise often targets the latissimus dorsi muscle in the back along with many other assisting muscles.