Single pole circuit breakers are the breakers with the capacity of 120V and 15-20A. They have one hot wire (live wire) and one neutral wire. They are used in fans, general lighting, vacuums, and electric irons.
On the other hand, double pole circuit breakers have the capacity of 240V and 20-60A. They have two hot wires (live wires) and one neutral wire. They can be used to supply two 120Vs or a single 240V circuit. Normally we use them in Air Conditioners.