What is the difference between SPST, SPDT, DPST, DPDT and 3PDT switch?

1. SPST (single pole single throw)
It simply connects/disconnects a single line (2 pins).

 ->o __ o->
 

2. SPDT (single pole double throw)
It switches one line to 2 different ways (3 pins).

->o __ o->
       o->
    
   
3. DPST (double pole single throw)
It  means  two  independent  SPST in one switch (4 pins).

->o __ o->
->o __ o->


4. DPDT (double pole double throw)
It  means  two  independent  SPDT in one switch. It allows true bypass switching (6 pins).

->o __ o->
       o->
->o __ o->
       o->
       

5. 3PDT
It is basically DPDT with one extra SPDT switch (9 pins). It allows true bypass + additional 
function (LED, ...)


Pole = number of switch contact sets
Throw = number of conducting positions

All of them are produced as either push/stomp, toggle or slide switch.