Mass drivers are a type of electromagnetic catapult, with applications for propulsion or orbital bombardment. In the latter instance, mass drivers collect meteoroids and other debris in space, condense the material, and catapult it toward the surface of a planet. Used in such a manner, mass drivers are considered weapons of mass destruction.
In the Pendragon timeline, the Klingon Defense Force used mass drivers to bombard the surface of Cardassia Prime, resulting in billions of deaths and incalculable ecological damage. (Star Trek: Pendragon)