A Brief Overview
The human nervous system is divided into two parts, the central nervous system and the peripheral nervous system. The central nervous system, CNS, is just the brain and spinal cord. The peripheral nervous system, PNS, includes the nerves and neurons that extend outwards from CNS, to transmit information to your limbs and organs for example. Communication between your cells is extremely important, neurons are the messengers that relay information to and from your brain.
Nerve cells generate electrical signals to transmit information. Neurons are not necessarily intrinsically great electrical conductors, however, they have evolved specialized mechanisms for propagating signals based on the flow of ions across their membranes.
In their inactive state neurons have a negative potential, called the resting membrane potential. Action potentials changes the transmembrane potential from negative to positive. Action potentials are carried along axons, and are the basis for "information transportation" from one cell in the nervous system to another. Other types of electrical signals are possible, but we'll focus on action potentials. These electrical signals arise from ion fluxes produced by nerve cell membranes that are selectively permeable to different ions.
image: http://sprott.physics.wisc.edu/images/neurons.gif