What do SPST and DPDT mean?
SPST stands for single pole single throw and DPDT stands for double pole double throw. A SPST switch is a simple on/off type switch and can be used for coil splitting and as a kill switch for the guitar.
DPDT switches typically have a total of 6 terminals. There are two isolated common terminals, usually located side by side in the center of the switch. Each of the common terminals can be switched between making contact with one of two other terminals, usually located in two rows on either end of the switch.
DPDT switches are available in many switching patterns; the most commonly used in this application are on/on, on/off/on and on/on/on. These all describe what connections are being made in the various switch positions. In the case of the latter two, a three-position switch is being described. DPDT switches are used most often for compound coil-splitting, parallel wiring of humbucker coils and series wiring of two pickups in a guitar but they have other uses as well. The on/on/on type is utilized for the series/split/parallel wiring option found in some of the Seymour Duncan wiring schematics. DPDT push pull pots are not available in the on/on/on configuration.