Runners' knee is a term that describes a number of different knee conditions rather than anything specific. Of course what makes a knee condition 'runners' knee' then is the fact that it's caused by the constant high impact caused by running regularly which often creates 'tracking problems' where the knee cap doesn't follow correctly over the joint and starts to grind the cartilage. If you have pain in your knee that's made worse by running, by walking down hills or down stairs and that's centred around the knee cap, then this can be described as runners' knee.
If you suspect you have such a condition then it's highly advisable to see a doctor who will probably advise you to rest your leg and to address the swelling by using ice, elevation and compression.
At the same time though it may help to employ some stretches and exercises to strengthen the tendons and muscle in the area and to get it moving correctly sooner. Here are a few you can try:
• Lie flat on your back with your knees up and feet flat on the floor as though you're about to do a sit up, then lift your bad leg out straight so you are pointing it out and holding your foot up at roughly a 45 degree angle. Hold for a short period then put it back down and repeat.
• In the same position, raise your leg to the 45 degree angle but then move it outwards drawing an arch and then back to the front again.
• Stand on one leg (your bad one) and then lean it forward slightly by bending so that it's at an angle before straightening out again.
• Stand in a forward stance with your bad knee in front and then lean your body forward to bend that leg slightly forward.
• Hold your foot behind you with your hands and feel a stretch on your quadriceps.
These exercises helped me greatly to rehabilitate myself ease the pain, but do make sure that you consult with your doctor first to ensure you're not doing more damage.