An ion drive is a type of propulsion that uses an electric field to accelerate charged particles and eject them at high velocities (in some designs, the particles approach the speed of light). This type of propulsion is used by many pre-warp civilizations to propel probes and lightweight spacecraft once they are in open space. The thrust produced here is quite gentle, often requiring from hours to weeks to reach full velocity.
In sublight vehicles, ion engines are fueled by power cells, liquid chemical reactants, onboard generators, or virtually any other device capable of providing sufficient power. The engine utilizes internal fusion reactions to produce a stream of highly-charged particles that are forced through the engine's exhaust port.
Ion engines are known to emit mildly radioactive byproducts, requiring onboard technicians to wear protective gear. Because of this radioactivity, it is common protocol for most ships to utilize other forms of propulsion during atmospheric flight (such as thrusters or anti-gravity drives and reserve use of the more powerful ion drives for orbital navigation.