A quick intro to ion propulsion from NASA JPL:

Ion propulsion isn’t something found only in science fiction. JPL engineer Mike Meacham looks at how ion engines are being used to drive NASA’s Dawn spacecraft through the solar system. Dawn is approaching dwarf planet Ceres in the main asteroid belt with arrival expected in March 2015. Previously, Dawn orbited Vesta, the second-largest body in the asteroid belt. Learn how ion propulsion works and why it’s the reason Dawn will be the first spacecraft ever to orbit two solar system bodies. More about Dawn at: http://dawn.jpl.nasa.gov/