There's an interesting article about dormers in the March 2006 Southern Living. "Add Distinction with Dormers" includes photos and drawings and building tips. The most common types are shed and gable and combinations; the half-gable, half-hipped dormer is also known as a clipped gable or jerkinhead, and is tricky to build. A side-bar tells you which dormer style looks best with which house style. For instance, the hipped dormer works best with Shingle, Prairie, and French Eclectic.