Pull-ups and chin-ups offer powerful upper body workouts. Often used as part of military and other physical fitness tests, pull-ups require a slightly different hand placement than chin-ups. Chin-ups sometimes are referred to as reverse grip chin-ups because you take your pull-up hand placement and turn it around.
One major difference in a chin-up and a pull-up is the direction of your hands. In traditional pull-ups, turn your hands so that your palms face away from you as they grip the bar, also called an overhand or pronated grip. (ref. 1 and 5) With chin-ups, turn your hands around so your palms face you. This grip is called an underhand or supinated grip. (ref. 2 and 5)
Where you place your hands on the bar is different with each exercise. Pull-ups require a wide grip, with your hands spaced slightly wider than shoulder-width. Placing them too wide, however, can keep you from targeting the correct muscles by decreasing the range of motion in your shoulder and elbow, so keep the spacing within a few inches of your shoulders. (ref. 1, click on "too wide" under comments) Chin-ups need a closer grip, with your hands shoulder-width or just inside your shoulders. (ref. 2 and 3)
Pull-ups and reverse grip chin-ups target the same major muscle: the latissimus dorsi. This muscle runs from your back around your side and up to the front of your shoulder on each side. The secondary muscles worked are different for each exercise. Both work muscles in your shoulders, arms, chest and back, but pull-ups target some chin-ups don't, such as the biceps brachii and the teres minor under your shoulder blade. Chin-ups use more of your pectoralis major -- the muscles in your chest -- than pull-ups. (ref. 1 and 2)
Changing your hand position alters which muscle groups you work with each exercise. Moving your hands closer in, such as 6 inches apart, when you perform a chin-up gets more biceps in the action, for example. (ref. 5) Moving your grip closer with pull-ups focuses more attention on the brachialis and brachioradialis on either side of your elbow. Increasing the intensity of each exercises is the same; either increase your repetitions, hold a dumbbell between your feet or knees, or wear a weighted dip belt. (ref. 1, 2 and 3)