I get a lot of questions about the difference between a chin up and a pull up.
To make it easy, the major difference when performing these exercises is the position of the hands on the bar. With the Chin Up the hands are placed on the bar in a supinated position at just about shoulder width, using a reverse grip. This can be seen in the picture above on the left. When performing a Pull Up the hands are placed in a pronated position a little wider than shoulder width apart using an overhand grip as seen above on the right.
The thing that seems to get confused the most with both of these exercises is how far you have to pull yourself up. With both the chin up and the pull up your goal is to pull your chest as close to the bar as possible, not your chin. Too many times I see people try to reach their chin over the bar or even worse touch their chin to the bar. This completely throws off the mechanics of the movement and actually makes it harder. Keep your torso nice and straight with your spine in a neutral position throughout the full range of motion. Pull your shoulders back and downward opening up your chest and engaging your upper back musculature. From here pull yourself vertical trying to bring your chest to the bar (think of trying to pull the bar right through your sternum), then control your descent all the way down to the full hang position. And Repeat.
Performing pull ups and chin ups this way will be much harder at first but will give you a much bigger bang for your buck in the long run. I guarantee your upper back strength and grip strength/endurance will sky rocket.