The primary goal of the compositor: Color matching a foreground element to a background plate during the compositing process,
(or making two images that were photographed seperately appear to be in the same lightspace when they are composited together).

Basic Color Correction:
matching the black and white points

matching the midtones

grayscale balancing

Lighting:

Match the quality of light:

Match the light direction:

Match the interactive light:

Color Matching:
Match fleshtones

Match the overall color of light

Match the specular highlights

Lensing / Grain:

Matching the Focus or Depth of Field:

Matching Lens Flares:

Matching Existing Grain: