Axons are nerve fibres that conduct electrical impulses from one neuron to another. They carry vital body information to and fro. Just as electricity is a flow of electrons, axons are a flow of our sensory information. When we touch or feel something, it instantly registers in our mind through fast travelling axons. Axons are usually pretty fast normally, but even so they are divided into two categories on the basis of speed: myelinated and unmyelinated axons.
This necessitates a discussion on what Myelin is. A layer of fat, myelin is often found in the form of sheaths. Within these sheaths are small gaps called Nodes of Ranvier. These nodes contain sodium channels which are good electric conductors. Myelin, however, is an insulator. This combination serves to further speed up the process of axon transmission, reaching speeds of up to 120 m/s. This is called saltatory conduction. Myelin sheaths vary in thickness, and the fastest conducting axons are associated with the thickest sheaths. In layman terms, myelin sheaths act as a highway for axons. They allow greater speeds to be attained for less energy expulsion. If a brain is opened up you would see some parts appear darker than other. The dark areas make up the grey matter, while the remaining portion is that of white matter. The white matter is the sensory part of our brain, and its apparent brightness is due to the high density of myelin sheaths.
This then is why myelinated axons travel faster. The answer lies in the simple laws of electricity conduction. A good conductor, surrounded by an insulator, is very much like an everyday wire. In the brain, that wire is the myelin sheath, which allows axons to conduct faster than normal.