Service, Main, or Electrical Panel
This is the metal box in your home where electricity enters your home (from the power company). The electrical panel allows electricity to be distributed to the outlets, lights and appliances.
Inside the panel is either a set of fuses (for older homes) or circuit breakers, which limits the amount of electrical current that goes to each circuit.
Typical circuits are rated for 15 or 20 amps, however, larger circuits (e.g., 50 amp) are used for major appliances (e.g., air conditioner, range, etc.).
A typical home will have 1-3 circuits per room, depending on the size and power requirements of each room.