Basically, any hunk of matter (I suppose it could be antimatter as well, but isn't that really beating a dead horse?) sent hurtling through space by a sufficiently advanced civilization at another planet or body at a very high speed (a high percentage of the speed of light). The resulting explosion could destroy whole planets, and possibly stars.

In the novel "The Killing Star," aliens, alarmed at the prospect of a spacefaring mankind, decide to prevent any future problems with us by lauching masses travelling at .92c at the earth, with two striking earth, 12 hours apart so that the surface would be completely sterilized. It is possible that the harnessing of antimatter would make this frightening invention a reality.