Dwarves use the Spoil skill to try to activate the "spoiled" condition on a monster. If it's successful, you will be able to use Sweeper on the monster once it's dead, to collect extra drops. Those extra drops are defined in the <spoil> section of the NPC droplists.
Plunder is basically the Goddess of Destruction version of Spoil+Sweeper. It works the same as before, but instantly grants the items to your character, instead of you having to manually Sweep after they die.