We can treat a switch as a box that hold some type of technology that when activated opens or closes a connection or a circuit. The switch is "n.o" or "no" meaning "normal open" when is not activated and the circuit is open. In contrast the switch is normally closed ("n.c." or "nc") when is not activated and the circuit is closed powering something without our intervention.
Since relays are switches, the terminology applied to switches is also applied to relays; a relay switches one or more poles, each of whose contacts can be thrown by energizing the coil (or activated) in one of three ways:
Normally-open (NO)
Normally-closed (NC)
Double-throw (DT or Change-over)
SPST – Single Pole Single Throw. These have two terminals which can be connected or disconnected. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity.
SPDT – Single Pole Double Throw. A common terminal connects to either of two others.
DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil.
DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil.
The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single actuator. For example 4PDT indicates a four pole double throw switch.
In conclusion we can describe a switch by his location in circuit (high side/low side) relative to load, by his default state (NO or NC), number of poles (contacts) and the number of throws (how the poles can be connected when the switch is activated).
