Relays
A relay is a switch, an electromagnetic switch. By passing a small current through a coil inside the relay, a magnetic field is created which pulls one switch contact away from or against each other. Relays come in many forms defined by the arrangement of contacts. There are SPST, SPDT, DPDT, DPST and more (see switches for more information).
Reed Switch Relays
An electronic reed switch relay is an enclosed glass tube with two switch contacts inside. The contacts are closed by a magnetic field.