A switch opens and closes to allow the flow of electric current or to stop it. There are many types of switches including, but not limited to manual switch you would have to turn on an off, switches reacting to pressure, heat, or to water or air.
Switches are designed to be either normally open (NO) or normally closed (NC). They will either make or break a circuit.
Example: A pressure switch is designed to sense unsafe operation of a furnace. It’s a NO (normally open) switch that closes on the rated negative pressure. When exhaust venting becomes plugged, the pressure the switch senses is not correct and breaks the circuit.
Wiring in the circuit connects to the contacts of the switch to move current through the circuit when the pole comes into contact with the contacts.
SPST – Single Pole, Single Throw
The switch is either open or closed like a furnace door switch and has one pole and one contact.
SPDT – Single Pole, Double Throw
Similar to the SPST, but with an extra throw (contact). The SPDT has a pole that can move to one of two contacts
DPST – Double Pole, Single Throw
Like the SPST but with an extra pole and contact. 240v appliances are controlled with one pole wired to L1 and the other pole wired to L2 with each pole being 120v.
DPDT – Double Pole, Double Throw
Similar to the DPST, but with an extra throw and extra contact per pole.