Also known as chromakey (or bluescreen), it used to be something that was reserved mainly for high-flying, blockbuster effects films. For the uninitiated, in theory it works quite simply: you paint parts of a scene you are photographing -- usually the background -- a consistent color that isn't found anywhere else in the frame (like a super crazy unnatural shade of green, AKA chromakey green), and in post-production you "key out" that color and replace it with whatever you like (like 1930s Taipei in the case of a period film, or an alien landscape in a sci-fi movie). As the technology and software have gotten easier and cheaper to use, however, greenscreen has been popping up in the most unlikely of places, from low-budget comedies to television shows, and the effect is often so seamless that many of these shots go unnoticed and unheralded (unlike in the old days). If you've ever watched Ugly Betty, for instance, there's a good chance you've seen a greenscreen shot without realizing it.
The following is a great little video clip that reveals how greenscreen and bluescreen are used in all sorts of productions, some of which are pretty surprising. It's amazing how often it's cheaper to do an effects shot than just shoot outdoors on the steps of some courthouse.
What do you think -- is it like eating lab-grown meat? Should we shoot everything in front of a greenscreen in a studio just because we can?