Chin-Ups a similar exercise to the Pull-Up, but there are some key differences. Aside from the obvious difference in grip, Chin-Ups use more shoulder extension rather than adduction, and due to the positioning they typically hit your biceps a lot more. Most people generally find Chin-Ups the easier of the bodyweight pulling exercises.
Chin-Ups can help build impressive strength and bulk in the back and biceps. Due to the nature of the activation of the Lats, the shoulder extensors, scapula rotators and depressors also get worked heavily.
1. Grab the pull-up bar with the palms facing your torso and a grip shoulder width apart.
2. As you have both arms extended in front of you holding the bar, keep your torso as straight as possible whilst pointing your toes. Think of the Hollow Body position. (See Fig. 1)
3. Pull yourself up until your chin is above the bar. Keep your elbows as close to your torso as possible. The body should remain as still as possible with your arms being the only thing moving. (See Fig. 2)
4. Squeeze your biceps at the top position then slowly lower yourself back to the starting hang position.

Fig. 1

Fig. 2

All the way through the lift ensure that your glutes, abdominals, lats and biceps are all tense, with the shoulders tucked back. The movement should be smooth and fluid.