Ah neurology, interesting stuff.

Resting membrane potential - the electric charge of the neuron when it is has not been stimulated (ie. its static state charge).  It is equivalent to -70mv.  When a neuron is at rest the resting membrane potential is a result of the separation between the extracellular potassium ions, and the intracellular anions across the cell membrane.

Depolarization - a change of the cell's membrane potential to be either positive, or less negative than it was to begin with.  If the depolarization is high enough it will cause an action potential.  The cause is the opening of sodium and calcium ion channels, which allows an influx of these positively charged ions into the nerve cell.

Repolarization - the returning of a cell to it's resting potential after an action potential, which results from potassium exiting the cell when the potassium channels open at the completion of the nerve impulse.  Usually during repolarization the membrane potential overshoots the resting potential becoming hyperpolarized, and then relaxes back down to its resting potential.

Never impulse - also known as an action potential.  Caused by the transmission of neurotransmitters across the synapse that bind with the receptor cells on the dendrites of the receiving neuron.  It causes the ion channels to open and depolarize the cell.

Refractory period - the period for which a neuron cannot re-fire after an action impulse.  It is equal to the time it takes for the neuron to reestablish its resting potential.