Here is a basic tutorial video of how to use green screen and then apply it on after affects. Green Screen is particularly used when you want to get rid of a the background or to add one with a specific photography of yours. For example a picture of a guy jumping and then you want to add an explosive background behind him or simply giving this feeling of height after he has jumped from a certain height. While using green screen techniques, specially in photography you can easily get the silhouette of the person in the photo with clean details, refine edges and no more problems of cleaning the edges again and again.