A pull-up is an upper-body compound pulling exercise. Although it can be performed with any grip, in recent years some have used the term to refer more specifically to a pull-up performed with a palms-forward position.
The term chin-up, traditionally referring to a pull-up with the chin brought over top of a bar, was used in the 1980s to refer to a palms-away (overhand/pronated) grip, with a palms-toward (underhand/supinated) grip being called a “reverse-grip” chin-up.
In later decades, this usage has inverted, with some using “chin” to refer to a pull-up done with a palms-backward position. In spite of this, “chin” is still regularly used refer to overhand-grip.
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 mostly targets the latissimus dorsi muscle of the back along with other assisting muscles.