Visual effects are often integral to a movie's story and appeal. Although most visual effects work is completed during post-production, it usually must be carefully planned and choreographed in pre-production and production. Visual effects are designed and edited in Post-Production, with the use of graphic design, modelling, animation and similar software, while special effects are made on set, such as explosions, car chases and so on.
Visual effects may be divided into at least four categories:
- Models: miniature sets and models, animatronics, stop motion animation.
- Matte Paintings, and stills: digital or traditional paintings or photographs which serve as background plates for keyed or rotoscoped elements.
- Live-action effects: keying actors or models through bluescreening and greenscreening.
- Digital animation: modeling, computer graphics lighting, texture, rigging, animating, and rendering computer-generated 3D characters, particle effects, digital sets, backgrounds.
Reference-
http://en.wikipedia.org/wiki/Visual_effects