NASA's Dawn spacecraft reached Vesta and Ceres using ion engines. Instead of the brief, powerful thrust associated with launch rockets, these engines produced a small push over long periods. The accumulated velocity change enabled an ambitious journey with efficient use of propellant.
In Dawn's engine design, xenon gas is electrically charged to form ions. An electric field accelerates them out of the engine; the spacecraft moves in the opposite direction. A power source, such as solar arrays, supplies the energy. Ion propulsion is one specific family within electric space propulsion.
This is demonstrated spacecraft technology. Its low thrust is unsuitable for launching from Earth's surface, and mission planning must account for available power and operating time. For readers examining future-engine claims, the mechanism provides a useful reference: the energy source and expelled material are identified. Electric power does not make an engine propellant-free. Keeping those distinctions clear separates established engineering from proposals awaiting demonstration.