Pullup is an exercise that works few groups of muscles.
Pullups are usually performed with two grips; overhand and underhand. Of course, there are other grips you may explore in later stage, such as hook grip (without thumb), three fingers and two fingers.

Continuing from previous topic, we are going to briefly introduce these few basic types of pullups:

Close grip – Hands are two thumbs apart and forces shoulder blades to pull closer.
Wide – Wider than shoulder, emphasis on latissimus dorsi (which gives you the distinct v-shape)
L pullups – Shoulder length grip, with lower body parallel to the floor. Requires lots of core strength and thigh strength.
Kipping – Utilizing the weightlessness of forward swing to explode the body upwards with a relax spine. All you need to do is master the timing of swinging and a strong grip.
Climber’s pullup – Progression to one arm pullups, whereby pulling to one side alternatively.
Muscle up – Easier to achieve after master kipping as this requires to pull oneself to chest over bar, rather than just chin over bar.

One of the best way to improve your overall pullups is practice all of them in one session, like such:
5 standard pullups
Rest 1 minute
5 close grip
Rest 1 minute
3 wide grip
Rest 1 minute
4 climber pullup

Repeat from top if you still have the strength.
Increase the reps when you finally get a good grip of it.

Photo credit to http://crossfitcc.com