CGI:Green and Blue Screens

 What is a green/blue screen?

A green screen allows film producers to create a whole new world which doesn't exist for their films to use as film locations. They can also re-create existing locations, which may be hard to access, without needing to be in that specific country or location e.g outer space. CGI allows a person's setting to change instantly, one minute they could be sitting in a chair in their living room and then be sitting on the same chair but this time in the middle of a jungle. This is because the image on the green screen behind the actor sitting on the chair changes.
An additional use of CGI is that it can change a person's appearance and transform them into aliens, monsters animated characters etc.This is what was done in the film Avatar to create the Na'vi humanoid species.
 IMAGE:(The actors practiced the scenes in the jungle, but they actually shot it in a greenroom)

Another advantage of using green screen is that it enhances the performance of action sequences done by stuntman, for example scenes were characters are jumping from high buildings or falling from high places. The actual action doesn't need to be carried out instead the actor or stuntman can just be held by ropes hanging from the selling or jump from a small height, and the green screen will make it look like they are actually jumping from an extremely high place or free falling in mid air.
To do this type of scenes a travelling mattes is used. 

How travelling mattes work:
 The blue screen allows two or three pieces of film to be combined.
The first thing that needs to be done is to film the entire length of the tall building, this will be used as the background plate. Then they will need to film the actress hanging from the ceiling waving her arms around (acting in a way that resembles falling) in front of a blue background.
Then the special effects department creates two mattes from the shot of the actress, using red filters, which causes the blue background to turn black. After this they are left with two mattes one with the actor's silhoutte
black and the other in reverse as well as the two initial shots. Combining all four pieces of film will generate the final piece of film.
Source

How it works:
A person acts in front of a green/blue background. Then a computer is used to replace every single green pixel with a pixel of another image, which might be a location which was already filmed or created. A problem with this is that is if the computer detects anything else which is green those pixels will also be replaced which includes the clothing of the person in the scene.
Another:Video on Green screen