The main parts of the basal ganglia are the caudate nucleus, the putamen, and the globus pallidus. Located on either side of the thalamus, the basal ganglia receive input from much of the cerebral cortex, particularly the primary motor and somatosensory cortices, as well as the substantia nigra. Much of the output of the basal ganglia is directed at the primary motor cortex, and the motor nuclei of the brain stem.
It is clear that the basal ganglia play an important role in motor control, but that role is not well understood. Damage to the basal ganglia produces severe deficits in motor ability, but the neuronal activation in the basal ganglia indicates that they do not specfically direct the muscular movements, nor are they involved in stimulus-triggered movement.
The connections between the basal ganglia and the substantia nigra suggest that the basal ganglia may play a role in the planning and initiation of self-triggered (i.e. voluntary) movement. This theory is further supported by the akinesia seen in people with Parkinson's disease.
Essentials of Neural Science and Behavior, Kandel, Schwartz, and Jessell, 1995, Appleton and Lange.
Physiology of Behavior: 6th edition, Carlson, 1998, Allyn and Bacon.