When they say an injured athlete had fluid drained from a swollen joint such as the knee, what is this fluid?

Synovial fluid: Usually this is synovial fluid. Synovial fluid's job is to lubricate and protect the joint. In the presence of injury, inflammation or infection, the synovial lining (part of the joint capsule), overcompensates by producing an excess amount of synovial fluid. Too much fluid can cause pain and tightness in the joint and restrict range of motion. With an acute injury, blood may also fill the joint.

Synovial fluid: This fluid is produced by the knee to help cushion it. We all have joint fluid but depending on what type of arthritis one has can depend on the internal fluid that is there.