Pullups are hard and for some people (like me) they are really really hard. This difficult exercise works out the lats, biceps, and middle back all with a single action. There are several variations such as reverse grip pullups (chin ups), wide grip, close grip, etc.. and they will all work out several muscle groups at a time.
If your thinking “This sounds great, but I cant lift my body weight…” don’t worry there is a little trick to get started doing pullups until your muscles get strong enough. Simply put a chain in front of your pull up bar and place 1 leg on it. The further away from the bar the chair is, the more difficult the exercise becomes, until you no longer need the chair. When using this method attempt to have good form so that the proper muscle groups are being worked.
A good instructional video on how to do a pullup is available here: BodyBuilding.com: How to Do a Pullup