SPST stands for “single-pole single-throw” and SPDT stands for “single-pole double-throw.” These abbreviations are often used for electrical switches, relays and microswitches.
Relays are classified by number of poles and number of throws. The “poles” of a micro switch, for example, are the terminals common to each path the current can take. Any position where a ‘pole’ can be connected is called a ‘throw’. The relays are also set as NO or NC. Read more here about what NO and NC means.
SPST is the most common form and has a ‘pole’ and a ‘throw’. Like a regular light switch at home. It is illustrated as follows:
The SPDT has a double throw and is illustrated as follows:
SPDT is good as a feedback signal as you can get both normal mode and mirrored mode from e.g.. a high voltage contactor. This always shows if the circuit is complete and the position of the main contactor when one pole is NO and the other NC. Both cannot be activated at the same time but follow the “break before make” concept.