Pull-ups are also known as chin-ups. They can be executed in different variations (overhand grip, underhand grip, neutral grip, wide-grip, close-grip, …) and belong, next to push-ups and a couple other exercises, to the supreme disciplines of bodyweight training.
Pull-ups are suitable as a back exercise for a wide upper body. Apart from this, pull-ups also engage the muscles of the arms to an extent that should not be underestimated. This means the biceps and the core muscles also profit from pull-ups.
Remember that with correct form and execution of the movement comes results!