Chroma key compositing, or chroma keying, is a visual-effects and post-production technique for compositing (layering) two images or video streams together based on colour hues (chroma range). The technique has been used in many fields to remove a background from the subject of a photo or video – particularly the news-casting, motion picture, and video game industries
A colour range in the foreground footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. The chroma keying technique is commonly used in video production and post-production. This technique is also referred to as "green screening" or "blue screening".
Take a look a few incredible movies that look nothing like you'd expect before special effects.
Click below for an interesting look at the power of chroma key in feature films.
Both films, Avatar (TOP) and Saving Private Ryan (LEFT) required the usage of multiple green screens in certain sequences. Red taped markers would assist the lining of CGI elements in post-production.