Switches are identified by the number of poles (circuits) they have and by the number of throws (ON/OFF/ON, for example). The most common combinations are:
SPST Single Pole, Single Throw: turns a single circuit on and off.
SPDT Single Pole, Double Throw: turns one of two circuits on.
DPST Double Pole, Single Throw: turns on two circuits at the same time.
DPDT Double Pole, Double Throw: controls two functions on two circuits.