Myelin is a substance that is naturally produced by the body and is made of lipids (fatty substances) and proteins. Myelin is present in the central nervous system, as well as the peripheral nervous system. Myelin is produced in two separate areas for each nervous system, however. Myelin for the central nervous system is produced in special cells which are referred to as the oligodendrocytes, whereas myelin for the peripheral nervous system is made in Schwann cells. Chemically these two types of myelin are different, but they serve the same purpose, regardless of which nervous system they're used to insulate.
If the nervous system could be compared to copper wiring, then it would be best to think of myelin as the insulation that is wrapped around the wires. The insulation the myelin provides has several benefits to the human body. First of all, it makes certain that electrical impulses travel straight down the nerves to their appropriate destination, and that the electricity doesn't discharge inappropriately. The myelin also stops outside influences from reaching the nerves, which could damage both the nervous system and surrounding tissues.
If the myelin sheath is damaged, or destroyed, then it leaves the nervous system vulnerable to a variety of problems. One of the most notable signs, particularly for suffers of multiple sclerosis, is the degradation of motor control. This is because the nervous system sends commands through the nerves, telling the muscles what to do. If those messages are lost because the insulation has been stripped from the wires, then only a partial message may come through, or no message at all. The more the myelin sheath is degraded, the more likely loss of any form of muscle or motor control becomes.