, a type of shot where live action in the foreground is combined with rear projection
in the background.
The classic interior shot of people in a moving vehicle is often done this way, although ambitious directors may do it with open vehicles as well, as William Wyler did with the chariot race in Ben-Hur.
Modern CG techniques and the advent of digital editing mean almost any shot can potentially be a process shot, but whether digital or analog, process shots are technically tricky. Not only do light sources, shadows and perspective have to match between scenes, but frame rate or color balance mismatches will also be noticeable, though subtle. There is a fascinating tidbit in the commentary to the Se7en DVD about how Fincher used a deliberate mismatch in frame rate between the interior camera and the (probably greenscreened) exterior camera to create a sense of disconnection and unreality outside the car in which Freeman and Pitt were leaving the first crime scene.