Single-pole switch controls just one circuit. A double-pole switch controls two separate circuits. A double-pole switch is like two separate single-pole switches that are mechanically operated by the same lever, knob, or button. The number of "poles" indicates the number of inputs, or put another way, the number of signals that can be switched. The number of "throws" is the number of "outputs" each input can be switched to.
If you are talking about a 120v light switch for a home, they usually are labeled based on the number of "ways". A normal, simple switch is on/off, single pole, single throw. A 3-way switch is single pole, double throw. A 4-way switch is 2 poles/2 throw, but they are wired so the connection is either straight across or criss cross.
A single pole light switch only has two screws and they are on one side of the switch. The switch only controls power flow one direction.
A double pole light switch has two sets of two screws one pair on each side of the switch. These switches are used to set up two and three way switching for lights, like when you have a switch at the top and bottom of a set of stairs. These switches control power flow in both directions. With the switch up one side of the switch allows current flow. With the switch down the other side of the switch allows current flow.