A reed switch consists of two magnetic contacts in a glass tube filled with protective gas: When a magnet comes close to a reed switch the two contacts become magnetized and attracted to each other allowing an electrical current to pass through. When the magnet is moved away from the reed switch the contacts demagnetize, separate, and move to their original position.

Reed switches are very reliable and last as long as 3 billion operations if used properly. They are designed for low currents. High voltages applied to the contacts may cause a spark, which may weld the contacts together. Please look at the following website:

I'd never heard of a reed switch before but there seems to be plenty of information on the Internet (I searched using google).

The basic idea is that you have two thin wires with a small gap between them. If you then bring a magnet near by, the wires get magnetized, attract each other, and close the circuit.