The belief and worship of gods which are evil and which actively seek to harm believers.
According to some definitions, Satanists are maltheists (although they usually say that they are worshipping the good guy and believe that God is the cosmic villain). Cthulhu and his buds in H. P. Lovecraft's Cthulhu Mythos would be considered maltheistic gods.
In real life, maltheism is quite rare, as most people prefer to worship something that doesn't want to feast on their bones, but there are a small number of maltheistic cults out there -- probably sharpening their knives and looking up your address right now...