Computer-generated imagery

Computer-generated imagery[1] (also known as CGI) is the application of the field of computer graphics or, more specifically, 3D computer graphics to special effects in films, television programs, commercials, simulators and simulation generally, and printed media. Video games usually use real-time computer graphics (rarely referred to as CGI)[citation needed], but may also include pre-rendered "cut scenes" and intro movies that would be typical CGI applications. These are sometimes referred to as FMV (Full motion video).
CGI is used for visual effects because computer generated effects are more controllable than other more physically based processes, such as constructing miniatures for effects shots or hiring extras for crowd scenes, and because it allows the creation of images that would not be feasible using any other technology. It can also allow a single graphic artist to produce such content without the use of actors, expensive set pieces, or props.
3D computer graphics software is used to make computer-generated imagery for movies, etc. Recent availability of CGI software and increased computer speeds have allowed individual artists and small companies to produce professional grade films, games, and fine art from their home computers. This has brought about an Internet subculture with its own set of global celebrities, clichés, and technical vocabulary.
Simulators, particularly flight simulators, and simulation generally, make extensive use of CGI techniques for representing the outside world.[2]

Source : www.wikipedia.com