These terms relate to the configuration of the contacts on a switching device such as an actual switch, contactor, or a relay. Here are some links to web sites that contain some information that can help to explain the various configurations, how they operate and what they can be used for.

Basic Electricity. Page down a ways in this to the switches section.

Shows some diagrams of SPST SPDT DPST and DPDT

See Section 22 / Switches

In the context of Analog Switches

Hope this helps
Michael Karas