Orbital bombardment was an evil method of bombarding planets from orbit, usually from large capital ships such as Star Destroyers. Although seldom capable of actually destroying a planet, sustained bombardment at the Base Delta Zero could eradicate all life on the surface, boil away some of the crust, and ignite the atmosphere, turning the planet into a sterile floating ball.
Orbital bombardment was often done at Sith parties for the amusement of Imperial officers, who never really understood how vast a waste of resources it was. On the other hand, the Rebels and New Republic objected to the concept on principle and thus did not employ it...for now.
Orbital bombardment should not be confused with orbital bombadment, a recreational activity enjoyed by Gungans.