A compositor is a person who is responsible for creating visual special effects through compositing.
Compositors work at the end of the production process. They are responsible for creating the final image using layers of previously created material. They use material from various sources. Such material could include rendered animations, special effects, 2D animations, live-action sequences, static background plates, etc. The compositor is tasked to creatively combine all these elements into the final image while ensuring that the project maintains visual continuity and style. To achieve this they enhance the lighting, match blacks and other color levels, add grain where required, add motion blur (if appropriate), create convincing shadows, make sure levels combine together seamlessly, use keying, rotoscoping, and mattes where necessary.
In post-production companies, some technical directors may take on the role of compositors.