The caudate is a subcortical brain structure that looks
something like a tail (as Webster's definition suggests); it's part of
the basal ganglia, along with the putamen, the globus pallidus, the
substantia nigra, and the nucleus accumbens.
The caudate plays an important role in the coordination of movement.
Even a simple movement--say, reaching over and picking up a can of
soda--requires a lot of information. You have to know where the can is
in space, which requires visual information; for that matter, you have to
know where your hand is, which requires proprioception. Then you have
to use spatial reasoning to figure out the best path between your hand's
current position and the soda can. You also have to know how to move your
fingers and your wrist so they're in the proper position; you have to
know how much force to exert, too, so that you can pick up the can
without crushing it. The caudate (and the basal ganglia in general) help
bring all this information together so that you can successfully reach out
and get your soda time and time again. Sure, all of this is (usually)
unconscious...but you still need the information.
Damage to the caudate causes problems with movement, including
bradykinesia and dyskinesia. In Huntington's chorea, the caudate
slowly atrophies, causing the jerky, involuntary movements that are
characteristic of the disease.