The pullup is a great exercise. It is a compound exercise - meaning it exercises more than one joint at the same time - and it calls on a large amount of muscle to accomplish. It's both a good test of strength (can you do one?), of endurance (how many can you do?), and a good way to build strength (unweighted at first, weighted later). There are a lot of variations - pullups (overhand grip), chinups (underhand grip), and all sorts of variations of grip width (from very wide spacing to overlapping hands) and position - even neutral grip, with palms facing. To break up the confusion, this blog will always refer to overhand grip pullups as pullups, underhand grip pullups as chinups, and everything else as a (whatever) pullup.
But what if even one pullup is too hard?
It can be hard to get your first pullup. Several good guides have been written. Here are a few I find useful.
Mistressing the Pullup - from the excellent Stumptuous.com, specifically aimed at women, but the advice is good for anyone. Remember, women and men aren't significantly different when it comes to training methods.
A Christian Thibaudeau article called Keep Your Chin-Up at T-Nation. Contains Not W/FS images.
Stronglifts has a good article about building up to your first pullup, and then beyond.
And finally, if you're doing a workout that calls for pullups but just can't get one, here are some pullup substitutions from Simplefit.org.