Illustration of an Synovitis and the capsule
Most of the moveable joints are contained within a 'capsule' which makes the space watertight. The inner lining of the capsule is comprised of a membrane of special cells (synovial cells) that secrete the lubricating fluid of the joint - the synovial fluid. If the joint becomes inflamed for any reason, it affects these synovial cells, which then produce excess synovial fluid. This causes the joint to swell and become painful.
Synovial effusion
The fluid that collects within the joint capsule is called an effusion. Because the capsule is normally like a baggy balloon, when it is empty movement of the joint is easy. But if it becomes tense with fluid it blows up like a balloon. The synovial membrane is rich with pain receptors, and inflammation of the synovium and a tense effusion can be very painful, with movement of the knee becoming difficult. The joint may feel warm and reddish in colour.
Synovitis in arthritis
Rheumatoid arthritis in particular is associated with recurrent synovitis, with multiple tensely swollen and painful joints. In osteoarthritis it still occurs but is generally less florid and only affects one or a few joints.