Patellofemoral pain, or patellofemoral syndrome, is characterized by pain under the kneecap, or to the inside of the kneecap. Pain often occurs when walking down stairs or sitting for a long time (it even has a nickname of "movie goers knee").
The actual mechanism of the pain is from the underside of the patella (knee cap) rubbing on the femur. If you imagine a valley, the patella is supposed to run smoothly in the valley. But if it is pulled out of the valley, the underside of the patella will rub on the femur, causing wear on the cartilage over time.