A generator is a device that converts motive force (mechanical energy) into electric power for use in an external circuit in the production of electricity. Steam turbines, gas turbines, water turbines, internal combustion engines, wind turbines, and even hand cranks are examples of mechanical energy sources. The Faraday disc, the first electromagnetic generator, was constructed by British physicist Michael Faraday in 1831. Generators supply approximately all of the electricity used by electric power systems.

An electric motor converts electrical energy into mechanical energy in the opposite direction, hence motors and generators are quite similar. Many motors can be mechanically driven to generate power, and they can often be used as manual generators.