What's The Difference Between A Single Pole Switch And And Double Pole Switch?

A single pole switch identifies the switch as a single switch with a single set of contacts.
example;  
SPST is a single pole, single throw switch with two connections.  It can be normally on (closed), or normally off (open);
o------/   /------o

SPDT is a single pole, double throw switch with three connections.  It has a normally on contact, and a normally off contact.  Sometimes toggle switches have a center position where neither is closed (ON-OFF-ON).
o------/_ /------o
  .   /------o

A double pole switch is actually two single pole switches with the same mechanical lever or knob.  It is used to switch two different circuits at the same time.
DPST is a Double pole, single throw switch pair that operate with the same toggle lever or knob;
o------/   /------o
 
o------/   /------o

DPDT is a double pole, double throw switch with two SPDT sets of contacts;

o------/_ /------o
  .   /------o
   
o------/_ /------o
  .   /------o

These symbols are not exactly the symbols used in proper schematics for switches.  Sometimes a set of relay contacts is designated with -----/  /----  but usually switches are done with something more like this;

  /
----o  o-----

but with text characters, the options to make schematic symbols are limited.