They practice medicine. Assess medical situations, order and interpret tests, diagnose/treat (including minor surgery) and if need be--prescribe medication. They can also specialize in any of a number of specialties and even fill residency positions in those fields.
There are any number of good web sites that might help you.
Tony: Smedly gave you a pretty good concise answer. PA's act as memebers of the healthcare team, acting as physician extenders. The role of individual PA's varies wildly. Some spend their days first assisting in surgeries. Some spend it doing histories and physicals. Some work ER's. A PA in a rural area may have a much wider range of allowable tasks, but all PA's will work under the supervision of a Physician.