What is 'Chroma Keying'???

We've all seen it - during the weather report, in a Star Wars movie, on a televised baseball game... Weird graphics slapped onto a moving image that normally wouldn't (or couldn't) be there. You've heard about green screens... But how do filmmakers use green screens to create a shot that takes their actors out of a studio and puts them into some exotic locale? How does the camera or the editing software know what to do with that green patch in your shot? Welcome to Chroma Keying...