Runner's knee is a term used to describe knee pain for runners. It is common in runners, but is not specific to that sport. The exact same thing can happen to basketball players or field jumpers. In that case, it would be called jumper's knee.
The pain is typically located just underneath or behind the knee cap. The simplest explanation for the problem is tendinitis. The overuse of the joint through repetitive movements or high impact sports caused inflammation of the tendon behind the knee cap. This inflammation causes pain.
Treatment depends on the athlete and severity. Icing and anti-inflammatory medications like ibuprofen are a good start. A reduction in the work load on the knee is recommended as well. Completely stopping the exercise would work too, but the athlete loses out on valuable training time.
Athletic taping can reduce the tension on the tendon and relieve pain. Over the counter runner's bands are available too. Good shoes with plenty of support will relieve pain, as will careful avoidance of running on very hard surfaces, such as concrete.