Cel Shading is a style of computer rendering that imitates the look of hand-drawn artwork and animation. In layman's terms, it replaces the shading gradient of conventional rendering with flat colors and shadows (as seen at right). The style was codified with the Sega Dreamcast game Jet Set Radio.
While the style is not exclusive to Video Games, it is often used there since this is the only way [...] [tvtropes.org]