DEF:
Pole: The number of distinct things that are switched, the common terminals
SP, Single Pole, 1 terminal switched
DP Douple Pole, 2 terminals switched
3P, 3 terminals switched
4p, 4 terminals switched
ST, single throw; two terminals total
an SPST toggle/slide switch is a Single Pole Single Throw switch with
two terminals total
A more concise way to write this is Off-None-On.
An SPDT momentary toggle/slide switch is Off-None-(On)
A 3PDT switch with the two ends momentary and the center off is (ON)-OFF-(ON)
This would have 9 total terminals on the switch.
You can concisely describe the switch in the latter notation, but nevertheless the other notation is still with us.
Relays are best described by a "Form X contacts". See
WikiAnswers - What is form C contact
The -NO and -NC are Normally Open and Normally Closed respectively.