Compositing is the art of creating a new image by combining images from different sources. It may envolve rendering multiple lighting components from a 3D model in Autodesk 3ds Max and combining the results in a new image using Adobe Photoshop.
This tutorial technique is one possible solution for overcoming what is a problem with all high-end 3D packages—excessive rendering times for generating acceptable presentation imagery. Instead, 3D compositing is a quick process for achieving an improved level of realism from relatively simple, quickly rendered image inputs.