Mandible - the lower jaw.

Condyle - the "ball" end of the mandible
or lower jaw.

Disc - a dense connective tissue pad that acts as a cushion between the condyle and the socket that it fits into. (Somewhat like the cartilage in your knees)

Fossa - the "socket" in the temporal bone that the Condyle fits into.

TMJ is an anachronism made up of the

T-Temporal bone, M-Mandible and J for the joint where they come together.