The vast majority of knee pain that doctors in primary care see is caused by chronic use/overuse injuries and knee cap tracking issues.
Adolescents during growth spurts can develop pain just below the knees that comes and goes and is worse with squatting, using stairs, or jumping. All these activities put strain on the site where the patellar tendon attaches and can cause a condition called Osgood-Schlatter lesion or Osgood-Schlatter disease.
Women of all ages (more than men, but not exclusively) are prone to a condition called chondromalacia, where the under surface of the knee cap begins to look like crab meat -- all flaky and ragged, instead of polished and smooth. What are the symptoms? Pain in the front of your knee, especially after prolonged sitting, is the classic one, sometimes referred to as the theatre sign.
Of course there are numerous tendon strains, cysts, and inflamed bursas (the bags of fluid that cushion between the bones and ligaments). Most of these respond to anti-inflammatory medicines and physical therapy, or possibly a simple drainage procedure done quickly in the office.
If you have knee pain, go see your family doctor, sports medicine doctor, or local orthopedic MD and find out what's causing your pain and what your options are for treatment.