green screening

"green screening" or "chroma keying" is a video editing post production method that nearly all types of media use. You can use a green screen to change backgrounds or slight changes in a video, most video editing software can do something called "chroma key" and this selects a color and removes it, creating something you can change easily, for example with a green-screen you could film a tv drama set in LA in your back yard. you can also use blue-screen for the same effect, let's say an actor was wearing green then a chroma key would pick up his green top if you were to chroma key it, so a blue screen would be more appropriate. in fact, a chroma key can be used for any color to suit the environment that you're in.

When the technique was first introduced into mainstream filmmaking it began as a painting on glass in front of the camera in black paint and shooting with a black background to create a mask around the subject, then taking that and treating it with the negatives to reveal just the subject you wanted from the mask and then using it to place on top of other footage to create the look of one shot.