Chin-ups challenge almost every single muscle in your back; this exercise is a contest of your body's power against it's own weight. The primary difference between chin-ups and pull-ups is that chin-ups are done with a supinated grip, palms facing in toward the exerciser. This sets up the mechanics so that chin-ups, while still primarily a back exercise, also provide a good arm workout, particularly for the biceps.
Latissimus Dorsi (Lats)
The primary mover in chin-ups is your latissimus dorsi, the large back muscle that stretches from your underarms to your waist. The lats are responsible for all strong pulling motions that move the shoulder joint, whether your upper arm is being pulled down toward your body or from the front of your body toward the back. When well developed, this muscle provides a characteristic V-shaped back, broad at the shoulders and narrower at the waist.
Trapezius (Traps)
Your trapezius muscle doesn't extend as far down your back as your lats do, and is more powerfully activated by exercises that position your elbows fairly high, such as wide-grip rows. The lower and middle portions of your traps do, however, exert some force during chin-ups.
Teres Major and Posterior Deltoid
The teres major performs much the same function as your lats; it's even known, at times, as "Lats' Little Helper". The posterior deltoid muscles help also, although to a lesser degree, with bringing the body up to the bent arm.
Biceps Brachii and Brachialis
Your biceps brachii and brachialis muscles (the latter is sometimes known as your lower biceps) are both powerful pulling muscles in the upper arm. They help flex your elbow as you bring your chin up to the bar. The standard chin-up hand position, with palms facing the exerciser, is ideal for strengthening the biceps brachii.
Rhomboids and Levator Scapulae
Your rhomboids are responsible for retracting (pulling together) and depressing (lowering) your shoulder blades. You'll work your rhomboids isometrically if you hold your shoulder blades in place, retracted and depressed, throughout the entire chin-up motion. If you release your shoulder blades at the bottom of the range of motion, then pull them down and in as you lift for the next chinup, your rhomboids will perform a more dynamic contraction. The levator scapulae helps rotate your shoulder blades downward and hold them in place.
Core
As with all exercises, your core muscles--the rectus abdominus, inner and outer obliques, transverse abdominus, erector spinae and sometimes your quads and glutes as well--should be activated to help keep your body stable throughout the range of motion.
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