Tandem breakers, sometimes called split breakers, provide two separate circuits in the space of a normal sized breaker opening. Every main electrical panel has a limited number of circuit openings available. The problem that many people encounter is that when the openings are all used up and you still need to add another circuit, such as maybe a few extra outlets in the garage or a home remodel, what do you do?
You could change the main electrical panel or you could "double-tap" a breaker, but this is not an approved application and can place to much strain on a circuit resulting in overheating, the potential for loose connections and ultimately this situation can become a fire hazard.
The answer that many have found is a tandem breaker. This type of breaker is the same size as any other breaker, but it has its unique differences. This breaker combines two smaller breakers built into one normal sized breaker. Each has its own breaker switch and the breaker snaps in just like a regular breaker. With this simple innovation, you can add a circuit and protect the circuit on its own dedicated line. Not necassarily an easier application, but much safer. In order to determine if your panel can receive a tandem breaker check with the panel manufacturer and review the legend in the panel. Many panels will list the type of breakers allowed and give schematics of where and how many tandem or split breakers can be used.