What exactly is a ganglion cyst?

Synovial cyst: Rupture of the synovium which is the cushion between the small bones of the wrist. The fluid that leaks out is then contained by the body and forms a cyst connected to the defect.

Fluid sack: A ganglion is a fluid filled cyst that connects with a joint often via a one way type passage. This makes it easy for fluid to move form your joint to the cyst, but not so easy the other way. They can vary in size rather quickly and often get bigger if you use the affected joint a lot. Rest and compression can help decrease their size. They are usually not painful unless pressed on.