As well as all the other knee injuries, there is a ligament that protects the inner side of the knee. This ligament can be simply over-stretched, or partially torn or in worse cases completely torn.
This ligament can be damaged by your leg being forced outwards as perhaps in football or by a twisting movement. The ligament may also be damaged by repetitive use for example swimmers doing the breaststroke often damage this ligament when they kick out as the knee is slightly twisted with each movement.
If you have immediately injured your knee you will feel pain on the inside of the knee. There may be swelling and if the swelling starts to cover the whole knee it could be that there is further injury to other parts of the knee. Medical advice is advised.
Usually pain is minimal at first and you only feel it when performing your sport, but later you start to feel the pain creeping in perhaps when simply walking or doing normal activities.
Always consult your Doctor before performing any exercise or treatment. All information on the common sports injuries web site is provided as general information only and should not be used for diagnosis or treatment of any sports injury.