Knee joint is a synovial type of completely mobile joint which bears total body weight. The smoothness of its movement is possible due to the smooth articular cartilage and synovial fluid. It itself has three sub joints as medial and lateral condular and patello femoral joint. It is supported and stabilised by multiple ligaments mainly ACL, PCL, MCL,LCL and menisci and bursae. It is one of the most essential joint for  walking. 
The common causes for knee pain are..
1. Trauma as accidents, twist injuries, fall causing damage to any of the above structures.
2. Degenrative conditions as arthritis, chondral defects etc
3. Infections, inflammatory conditions as Rheumatoid arthritis
4. Osteoporosis, stress fractures etc.
  So to treat knee related pains, proper clinical examination and diagnosis and right investigation is important. Medications, physiotherapy and in needful conditions, Surgery is essential.