The mechanical relay categories below are divided based on the input coil voltage. Relays are available with a variety of specifications:
Abbreviations are as follows:
NO - Normally open - this is a relay that will not pass power unless a voltage is applied to its coil.
NC - Normally closed - this is a relay that will pass power unless a voltage is applied to its coil.
NO and NC relays are typically "single throw" (ST), meaning that they either open or close the circuit. A "double throw" (DT) relay closes one circuit at it opens the other.
These devices are also classed by how many "poles" they switch. A "single pole" (SP) relay switches only one conductor. A "double pole" device switches two conductors.
Therefore, a DPDT relay, for example, refers to one that is "double pole, double throw", and that means that the relay can manage two conductors and connect those two conductors through a choice of two circuits.