Short answers, pulled from the story.
Compositing is the process of combining visual elements from separate sources into a single image to create the illusion they belong to the same scene. It covers techniques ranging from chroma key shooting with blue or green screens to physical glass shots and multiple film exposures. Most compositing today is achieved through digital image manipulation.
Georges Méliès is credited with early compositing in the late 19th century through his trick films. He used multiple exposure techniques, including a seven-fold exposure in his 1900 film L'homme-orchestre (The One-man Band).
Compositing software designates the blue or green backing color as the area to be replaced. Every pixel within that color range is then swapped with a pixel from another image, aligned to appear continuous with the original frame. Blue and green are used because they sit far from human skin tones, allowing precise isolation.
A glass shot positions a large pane of glass between the camera and the filming location. A scene is painted on most of the glass, leaving a clear area where live action is photographed through it. The plantation approach in Gone with the Wind is a classic example, with buildings and fields painted while actors moved through the clear portion.
The four basic compositing methods are digital compositing, physical compositing, multiple exposure, and background projection. Background projection includes both front projection and rear projection variants.
Digital compositing eliminated two major problems of the traditional five-strip optical process. The old system could allow film strips to drift out of registration, creating halos and edge artifacts. Film also loses quality with each copy generation, while digital images can be duplicated without any quality loss, making multi-layer composites practical.