Pull-ups are great for upper body, back and abdominal muscles. The basic pull-up is done from a "dead hang" where you grab the bar with an overhand grip and extend your arms until they are straight and then (while you stay totally still) pull your self up till your chin is at the bar and then lower your self down. This action counts as one repetition.
Doing a Pull-Up[]
How to do a pull-up if you cannot do a pull-up