myelin sheath

The purpose of the myelin sheath is to allow impulses to transmit quickly and efficiently along the nerve cells. If myelin is damaged, the impulses slow down.

nodes of Ranvier interrupt the insulation at intervals, and this discontinuity enables impulses to jump from node to node in a process known as saltatory conduction.

 
Schwann cells are the supporting cells of the PNS.  Like oligodendrocytes schwann cells wrap themselves around nerve axons, but the difference is that a single schwann cell makes up a single segment of an axon's myelin sheath.

In addition to creating the myelin sheaths of PNS axons, Schwann cells also aid in cleaning up PNS debris and guide the regrowth of PNS axons. 

Hope this helps.

The myelin sheath around most vertebrate axons provides a layer of insulation that prevents the loss of the electrical signal from an action potential. Myelin is lipid rich substance that does not conduct electricity well. Because of the insulation provided by the myelin, action potentials can travel much more rapidly down myelinated axons than down unmyelinated axons. Myelin also isolates axons from one another in the white matter of the brain preventing the short-circuiting of nerve signals in the central nervous system.