Circuit breakers are the safety switches that protect all of the electrical circuits in your house. There are two types of standard breakers: single-pole and double-pole. Single-pole breakers are rated for 120 volts and 15 or 20 amps. They control standard lighting and outlet circuits as well as some appliance circuits in the house. The breakers themselves are relatively narrow and occupy a single slot in the home's breaker box. Double-pole breakers, on the other hand, are typically rated for 20 to 60 amps and supply 240-volt power to large appliances, like electric dryers and ranges. They're "double-wide" breakers that take up two slots in the breaker box and have a toggle switch that's twice the length of the toggle on single-pole breakers.
Inside your breaker box or main service panel are energized metal plates, called "hot" bus bars. Each pole, or connection point, on the bars, carries 120 volts of electricity. When single-pole breakers are installed, they snap into one pole to receive 120 volts. Double-pole breakers snap onto two poles for a total of 240 volts. Circuit wiring that connects to double-pole breakers contains two "hot" wires. Each of these connects to a terminal on the breaker and is protected by half of the breaker. If a fault or other problem occurs along one of these wires, the corresponding half of the breaker will trip. This causes the other half of the breaker to trip at the same time because the two halves are tied together by the single breaker bar or toggle. This effectively shuts off the connection to both bus poles, shutting down the entire circuit at once.
If you open the door to your service panel and see a breaker that is single-width but has two small switch toggles, either inline or side by side, it's probably a "cheater" breaker. Also called a tandem, slimline, or twin breaker, a cheater is a double breaker that takes up the space of a single-pole breaker. Unlike a double-pole breaker, which serves a single 240-volt circuit, a cheater serves two 120-volt circuits; it cannot be used to supply 240 volts to a single circuit. A different type of doubled-up breaker is a "quad breaker," which serves two 240-volt circuits but is the same width as a standard double-pole breaker.
The use of a cheater breaker isn't meant to insinuate that your electrician was cheating—that's just a nickname—but these breakers must be installed properly to be legal and safe. First, the panel must be designed to accept cheater breakers, and second, the cheaters must be installed in the appropriate slots. On many panels, only some of the slots are suitable for cheaters. Cheaters are often used when space in the panel is scarce and someone wants to add two 120-volt circuits or to add room for a new 240-volt circuit elsewhere on the panel. If you want to know whether your panel can accommodate cheaters, call an electrician. By the way, modifying a cheater breaker to fit where it doesn't belong is cheating, and is downright dangerous.