The idea behind rotoscoping animation is using the real movement of people and animals as the basis of realistic animation. The old animated movie, Gulliver’s Travels was partially made with rotoscoping. As you watch the animated clip, pay attention to the difference between how Gulliver moves, who was rotoscoped, and how the rest of the characters move, none of which were rotoscoped.
The rotoscoping was done by Max Fleischer who was also the inventor of rotoscoping animation. A real man was filmed making the movements that Gulliver would have to make, and Max Fleischer traced those movements frame by frame as his basis for animating the character of Gulliver. The Disney film, Snow White was also rotoscoped by the animators on staff there.
Nowadays rotoscoping is done with the aid of computers, and is much less tedious with some methods. In the case of the second video, rotoscoping is used to separate the person from the background so another background can be substituted. The beginning of the video does not show you how using a computer is easier, but then the film maker will show you how to use the rotoscoping brush. After Effects, the program he uses is made by Adobe.
The demonstration shows how to change the background on a video without using a green screen. With green screen your subject would not be able to wear green, because the background shown on the green screen would also show up on the green item of clothing. Blue is sometimes used for this very reason, and the background is shown on a blue screen, with no blue clothing on the subject.
Green screen is most famously used when a weather newscaster stands in front of an animated weather map and points to it without interfering with the images on the map. He or she does not actually stand in front of the animated map, but instead in front of a bright green or blue wall or screen, and he or she looks at another monitor, which is showing what is on the animated map, so he or she can gesture to the weather front in question.
If you are still wondering how green screen works, I think it has something to do with using the uniformity of the green or blue background. This is so that in the video channels for making television or movies, the green or blue is identified as though it is transparent or part of the Alpha channel. This is instead of it being part of the green or blue channels from RGB (Red, Green, Blue) light. Because of this transparency, the background of a scene can be changed to whatever you like. I am not too sure how it really works though. Since it is part of movie magic, it is not explained online too well, but it also is called Chroma-Keying..