Sodium channels are special "gates" within the membrane that separates nerve cells (neurons) from the surrounding environment (the extracellular fluid). These channels are opened or closed depending on electrical or chemical stimuli. When they are open, they let charged sodium particles (ions) pass through them.
Sodium ions (Na+) and potassium ions (K+) flow between the neurons and the extracelluar fluid to set up the state of electrical charge potential when the neuron is at rest (the resting potential) and to release that potential when the neuron is sending a nerve impulse (the action potential). Potassium ions flow through other special gates called "potassium channels".
Sodium channels are opened a the moment the action potential occurs to let the positively charged sodium ions flow from the extracellular fluid into the neuron. How sodium and potassium channels are involved in nerve transmission is dealt with in the section on the action potential.
There are two main types of ion channel, voltage-gated channels and chemically-gated channels. Voltage-gated channels open or close in response to the surrounding electro-potential while chemically gated ones open and close in reponse to chemical stimuli. Sodium channels are almost all voltage-gated.
See Action Potential.
Sodium Channel Links:
Ion Transport Tutorial
Basic Neuron Physiology