[maan17]

The illusions or tricks of the eye used in the film, television, theatre, videogame, or simulator industries to simulate the imagined events in a story or virtual world are traditionally called special effects (often abbreviated as SFX, SPFX, or simply FX).
Special effects are traditionally divided into the categories of optical effects and mechanical effects. With the emergence of digital film-making tools a greater distinction between special effects and visual effects has been recognized, with "visual effects" referring to digital post-production and "special effects" referring to on-set mechanical effects and in-camera optical effects.
Optical effects (also called photographic effects), are techniques in which images or film frames are created photographically, either "in-camera" using multiple exposure, mattes, or the Schüfftan process, or in post-production processes using an optical printer. An optical effect might be used to place actors or sets against a different background.

source: Wikipedia