The refractory period in a neuron occurs after an action potential and generally lasts one millisecond. An action potential consists of three phases. Phase one is depolarization. During depolarization, voltage-gated sodium ion channels open increasing the neuron's membrane conductance for sodium.
The refractory periods are due to the inactivation property of voltage-gated sodium channel and the lag of potassium channels in closing. Voltage-gated sodium channels have two gating mechanisms, opens the channel with depolarization and the inactivation mechanism that closes the channel with repolarization. While the channel is in the inactive state it will not open in response to depolarization. The period when the majority of sodium channels remain in the inactive state is the absolute refractory period. After this period there are enough voltage-activated sodium channels in the closed (active) state to respond to depolarization. However, voltage gated potassium channels that opened in response to depolarization don't close as quickly as voltage gated sodium channels return to the active closed state. During this time the extra potassium conductance means that the membrane is at a higher threshold and will require a greater stimulus to cause action potentials to fire. This period is the relative refractory period.
Adapted from the Wikipedia article Refractory period (physiology), under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki