SPST

"Single Pole, Single Throw". You can visualize it as a simple on-off switch: There are two terminals, and they are either connected or disconnected.

There are two basic types of SPST switches: Momentary and on-off. Momentary switches are specified by whether they're either Normally Open(NO) or Normally Closed(NC). Closed circuits are "ON" and open circuits are "OFF". The NO variety require that whatever activates the switch be present for it to close a circuit, while the opposite is true for NC. Example of these include doorbell buttons, the buttons used for user input on electronics, or magnetic switch in alarm systems.

On-Off switches can be "flipped" to stay in either the open or closed circuit state. An example of these are common light switches.

Typical symbols for SPST switches.