This can be as simple as a flashlight cell connected through two wires to a light bulb or as involved as the Space Shuttle.
Electrical systems are groups of electrical components connected to carry out some operation.
For example, a subway rapid transit electrical system is composed of the wayside electrical power supply, wayside control system, and the electrical systems of each transit car.
Inside of each transit car there are also subsystems, such as the car climate control system.
The following would be appropriate for the design of a moderate to large electrical system.