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Long process which often projects to distant regions of the nervous system. The axon is the main conducting unit of the neuron, capable of conveying electrical signals along distances that range from as short as 0.1 mm to as long as 2m. Many axon split into several branches, thereby conveying information to different targets. Many neurons do not have axons. In these axonal or amacrine neurons all the neuronal processess are dendrites. Neurons with very short axons are also found. 

The axons of many neurons are wrapped in a myelin sheat, which is composed of the membranes of intersticial cells and is wrapped around the axons to form several concentric layers. The myelin sheath is broken at various points by the odes of ranvier, so that in cross section it looks like a string of sausages. The myelin protects the axon, and prevents interference between axons as they pass along in bundles, sometimes thousands at time.

The cells that wrap around peripheral nerve fibers - that is, nerve fibers outside of the brain and spinal cord - are called Schwann cells (because they were first described by Theodor Schwann). The cells that wrap around axons within the central nervous system (brain and spinal cord) are called oligodendrocytes. The axon, with its surrounded sheath, is called a nerve fiber. Between each pair of sucessive Schwann cells is a gap of a node of Ranvier.

Maybe you have heard the term "grey matter" for the brain; there is also "white matter". In a section through the brain it is easy to see both grey and white areas. The cortex and other nerve centres are grey, the regions in between, white. The grey coloration is produced by aggregation of thousand of cell bodies, while the white is the color of myelin. the white color reveals the presence of bundles of axons passing through the brain, rather than areas in which connections are being made.