Positional Differences: A pull-up is performed with a pronated grip (palms facing forward), usually with the hands spaced farther apart. (See Figure1)
A chin-up is performed with a supinated grip (palms facing backwards), usually with the hands closer together. (See Figure 2)
Anatomical / Workload Differences: The pull-up works the teres major, lower and middle trapezius, serratus anterior, latissimus dorsi (due to grip width), and brachioradialis more.
The chin-up works the biceps, forearm flexors, pectoralis major (costal head / “lower chest”), and pectoralis minor more.
All other muscles are worked roughly the same by both exercises.
As an overly-simplistic but helpful shortcut… you can think of the pull-up as engaging the muscles on the back of the body more (posterior chain) and the chin-up as engaging muscles on the front of the body more (anterior chain).
Relevant Research Articles:
1) Kinematic and electromyographic comparisons between chin-ups and lat-pull down exercises
2) Surface electromyographic activation patterns and elbow joint motion during a pull-up, chin-up, or perfect-pullup™ rotational exercise
Dr. Gains 
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