The human cerebellum is divided into several major sections. In the center is the vermis. The vermis is responsible for assisting in keeping you balanced (balance is a complicated function that requires multiple parts of the brain, like the superior colliculi, for example). On either side of the vermis are areas called the intermediate zones; these regions fine tune your movements, making them more fluid. If you've ever seen a Kung Fu master practicing his art, you've seen an excellent example of the intermediate zones hard at work. On either sides of the intermediate zones are the lateral zones. The lateral zones act in making complex ballistic actions possible (hitting the tennis ball with the racquet, throwing a flaming hamster across the room, etc.). Encasing all of that is the flocculonocular lobe (also aids in balance).
What happens if your cerebellum gets lesioned ("lesion" is neuroscience speak for "cut" or "damaged")? That depends on where the lesion occurs. Damage to the vermis or flocculonocular lobe will result in ataxia (loss of balance). On the other hand, a lesion to the intermediate zones causes asynergia (decomposition of movement into jerky, choppy motions). Dysmetria (underestimating the distance to an object) and hypermetria (overestimating the distance to an object) result from, yep, a lateral zone boo-boo.
Please note that the cerebellum does not actually synapse to the spinal cord. In English, the cerebellum has no direct control over muscle movement. Instead, it is responsible for receiving and interpreting spatial data.
Also please note that no hamsters were set on fire during the course of this WU. I have nothing but the utmost respect for hamsters; I would never, ever, ever light one up and hurl him across the room...