Propagation along a myelinated axon at the nodes of Ranvier. The action potential "jumps" between the nodes. Myelin covers the axon in regions between the nodes, reducing the leak out of the membrane, and also reducing the capacitance of the membrane so that less charge is needed to maintain a given membrane potential. Less charge needed means that the membrane can charge more quickly.

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Another view of saltatory condction along a myelinated axon

Propagation of the action potential along a non-myelinated axon. The action potential at each point along the membrane boosts the depolarization at neighboring points so that channels at adjacent regions of the membrane can reach threshold. This amplifcation travels along the membrane propagating the action potential. Action potentials do not move backwards because activity in the backward direction runs into the refractory period of the already-active channels.