Instead of burning chemical fuels, ion engines are electrically charged and work by ejecting positive ions from the back of the rocket, propelling the spacecraft forward. The thrust is small though; in fact it's the same as the pressure of a piece of paper on your hand.

          However, the ion engine is incredibly efficient, allowing the fuel to last far longer than in conventional rockets, making these spacecraft lighter and cheaper to launch.               

           Ion engines are already being used in multiple satellite systems, and in 1998 NASA launched a probe called 'Deep Space 1' to test the usefulness of ion engines. Due to its increased efficiency, Deep Space 1 has now been in space for longer than all the conventional space -rockets ever made put together!

            Future missions for these ion engines could include comet sample returns, observations of the rings of Saturn, and a landing on Jupiter's moon, Europa. Their light mass means that ion powered spacecraft can gradually accelerate to extremely high speeds. For example, if an ion engine was chosen for the forthcoming Rosetta comet mission in 2011, it could rendezvous with Comet Wirtanen and return samples back home in approximately five years. Conventional rockets would take nine years just to catch the comet, let alone return with the precious bounty.

Source: Outbound, by George Mason  

Picture: http://space-power.grc.nasa.gov/