Green screen technology works with the use of computers which tunes into the green color and replaces every green pixel with a pixel from another image. A camera is aimed at the green background and sends this information to a switcher or a chroma key device. The camera outputs a very high voltage from the image; once the output of green from the camera matches that of the chroma key device the green from the camera "disappears." This allows for a matte or some other video to be superimposed on the background This technology can be implemented using any color, but green is the easiest for most modern cameras to pick up, which helps the editor to create a much clearer image. Other colors are also much more common in wardrobes and lighting so are avoided because they result in a patchy image. The goal is to get a solid background which the editor can then remove with software
http://www.instructables.com/answers/how-does-a-green-screen-work-Why-is-it-green/
One of my favorite TV shows, Grey's Anatomy, was featured a few times in the YouTube video and I never would have guessed it was a green screen. Although now it seems like it should have been obvious, using a big blank screen as your background would probably be a lot cheaper than lighting a giant ship on fire. However my absolute favorite example of the green (or blue) screen technology was in Avatar, like many others will most likely agree. Not only did they use a blue screen, but the actors also used a special type of blue suit so their characters too were completely computer generated!
http://blogs.howstuffworks.com/2009/03/06/how-green-screen-works/http://www.instructables.com/answers/how-does-a-green-screen-work-Why-is-it-green/