Jumper's knee (patellar tendinitis) is a condition which arises from constant stress being put on the patellar tendon, resulting in a fraying of the tendon.
The patellar tendon is around two inches in length, and it slides about 1/4 of an inch when the knee is extended. When you run, 1500 lbs of force pass across this tendon. When you jump, the force increases to a whopping 2500 lbs.
This condition is common among leaping atheletes, and is thus referred to as "Jumper's Knee". Why some people develop the condition while others don't is unclear. However it is more prevalent in male atheletes, and seems to be unrelated to any sort of knee trauma.
The most noticable symptom of Jumper's Knee is pain. The pain will generally develop over a two to three week period. The pain is usually at its worst in the mornings, right when the sufferer gets out of bed. If one were to attempt to engage in a sport while affected by the condition, the pain would subside as the joint warmed up, but would increase substantially after the game ended. Jumper's Knee is usually painful to the touch, and often appears swollen.
The best treatment is rest, coupled with daily application of an icepack and the use of anti-inflammitory medication. In severe cases, a cortisone injection may help, and in a particularly difficult case one might have to resort to wearing a cast. While this wouldn't directly aid in the healing process, it would serve as a radical but effective measure to force the patient to stay off the leg and get some rest.