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Typically a graphical style where there are one two colors. (A bit can be either on or off, so two options) Commonly black and white, but can be any combination depending on the style the creator is trying to accomplish. 

Set in a fictional universe, often (but not always) with fantastic or supernatural elements, alternate worlds, magic and monsters.

Science-fiction. Commonly including things like aliens, advanced technology, and settings of outer space or distant planets.