Blue Screen

A basic answer is having talent stand in front of a blue screen and then use a device (hardware or software) to remove that color and replace it with another image or video source. This gives the illusion of the person being somewhere else, and it's a more common technique than you might realize. Not only is the weather man being keyed in front of a blue screen, but more and more often field correspondents and even newscasters are blue screened into virtual sets. Sometimes blue screen is also called chromakeying, more general term which covers keying on many colors including green and red. blue screen has been the traditional choice of colors because it is most diametric to human skin tones. Historically film and video cameras have been more sensitive to blue to counter the excess of green in the human eye's visible color gamut, but this is not so true any more. A blue screen is sometimes forsaken for green when control of what the talent wears is out of the producers hands (in a field environment perhaps), because people tend to wear more chrominant blue than green making for a harder blue screen chroma key. Green also tends to yield a less attractive spill, because green wavelengths have more candela the spill off of a green screen backdrop will be brighter and more noticeable than a blue screen backdrop.

 Blue Screen Backdrops

Getting a good even color behind the talent is the first step in blue screen, this usually requires a chromakey backdrop in the form of cloth or paint. Some blue screen paint can be picked up at hardware stores or lighting places like Studio Depot (Burbank, Ca), blue cloth can also be found many places. Some companies sell blue screen pop up blue screen backdrops which travel easily and popup like a windshield shade.
Another alternative is Chromatte, a unique retroreflective material which is gray to the eye, requires little or no light, and casts no spill on the talent because the material is gray to the naked eye. But to the camera it sees it as a perfect blue or green. This is because of the Litering, a ring of LEDs around the lens which casts blue or green light onto the Chromatte. Chromatte is also very portable in the form of a Chromaflex, a 7x7 popup which folds down into a 3 foot hoop. Chromatte is superior to blue screen in many ways.