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.