A paperclip maximizer is an agent that desires to fill the universe with as many paperclips as possible. The use of paperclips in this example is not important, but serves as a stand-in for any values that are not merely alien and unlike human values, but seem, to a human, to be arbitrary and worthless. It is usually assumed to be a superintelligent AI so powerful that the outcome for the world overwhelmingly depends on its goals, and little else.
A paperclip maximizer, although not explicitly malicious to humans, is nevertheless only slightly less dangerous than if it was. Contrary to what some may think, it does not "realize" that all life is precious, despite its intelligence. It sees life the same way it sees everything else that is made of atoms -- as raw material for paperclips. The nonhuman values that the paperclip maximizer holds make it an example of Unfriendly AI.