There are a few forms of green screen technology. The simplest is when you watch the nightly news, the background is simply replaced with whatever image or video they want to show at the time. You get more complicated with a movie like The Avengers, the entire set they act in is a green screen with the entire environment and other effects put in later post production.
This new technology in the video though can simulate an entire space IN REAL TIME! If you didn't know the entire room was being generated as he walked around in it, you'd think it was real