In Star Wars, a light-blue "laser" that disrupts ships' power grid (that is, it disables them). This is useful for capturing ships.
In playing the more complex Star Wars flight simulator games (X-Wing, TIE Fighter, X-Wing vs. TIE Fighter, etc.), ion cannons merely reduce an opponent's shield, and only disables power systems once shields are out. In "real life" (the movies themselves), however, they seem to work through shields. The best (and only?) example of this is at the start of Episode 5: The Empire Strikes Back, as the rebels are fleeing. | 
