A switch is the most common circuit control device. Switches usually have two or more sets of contacts. Opening these contacts is called "break" or "open" the circuit, Closing the contacts is called "make" or "completing" the circuit.
Switches are described by the number of Poles and Throws they have. "Poles" refer to the number of input circuit terminals while "Throws" refer to the number of output circuit terminal. Switches are referred to as SPST (single-pole, single-throw), SPDT (single-pole, double-throw), or MPMT (multiple-pole, multiple-throw).