Relays

Relays are electromechanical switches that can be actuated by an electrical signal rather than by manually opening or closing the switch. Relays figure prominently in the history of electronics, especially in the development of the telegraph network. Today relays still serve niche applications where total isolation between electric circuits is required and where power consumption is not a primary concern.
There are several typical relay configurations, but it comes down to poles and throws. In the context of a relay, a pole is a switch, and a throw is a possible connection. The simplest relay is a single-pole single-throw (SPST), which has two ports A and B. The single pole indicates there is one switch, and the single pole indicates there is only one possible connection (A to B). When the SPST is off, A is electrically disconnected from B. When SPST is on, A is electrically connected to B. A single-pole double-throw (SPDT) relay has three ports: A, B, and C. The SPDT relay may connect A to B when in the off position and A to C when in the on position. The double-pole single-throw is really just two SPST relays that are actuated by the same control signal. Similarly, the double-pole double-throw relay is basically two SPDT relays that are actuated by the same control signal.
In an electromagnetic relay, the relay is actuated by an electric current. The current produces a magnetic field that moves the relay's armature that either breaks or makes a connection. High power relays use a double-break design that opens the connection in two places, whereas smaller relays are typically single-break.