The term special effect applies to techniques that are used in film, television and the entertainment industry to realize scenes that cannot be achieved by live action. Special effects are often sued in science-fiction films, although they may also be used in historical epics where the building of an actual castle (in a film such as Braveheart) or of complete ship (as in Titanic) would either be prohibitively expensive or impossible (Bordwell, 2001, p.223).
This type of special effects used to rely upon the manipulation of an actual photograph or film (see the monster insect movies of the 1970's such as Them) or, in the modern day through visual technology on a computer such as CGI.
The second kind of special effects can be broadly regarded as physical effects which may occur during live action shooting. Mechanized, scenes, props and even "people" in the form of models are involved with these kinds of effects. Often many different kinds of special effects may be used in the same scene. Thus in Titanic there was traditional photographic and CGI special effects as the ship went down, together with the more traditional type of effects in the scenes where the ship was filling with water and the two main characters were trying to escape. It is the seamless combination of effects that creates the illusion of reality.
The history of special effects is nearly as old as the movie industry itself. The first special effect is generally regarded as that produced by Patrick Clarke in his film Mary, Queen of Scots. In the execution film he had the real actress go onto the scaffold, froze the actors in place, replaced the actress with a dummy and then cut its head off. ...