Green screen is another form of compositing. It allows a production to shoot an Actor in production and then replace the background(green screen) with something else. When filmming green screens, the background needs to be a flat color. This can be trick because of how you place your actor against the green screen. If the actor is too close to the background it can cause shadows. If a light is too close it can cause a highlight on the screen. Either add/remove more lights from the scene to get rid of shadows and highlights or reposition the lights.
Key Light is a program that is specific
keying (removing) the background; However, since we don't have access to a program like that the best alternative is After Effects. Using Color Key is the easiest way to get the look that you want. After Effects main function was not in chroma key but there are methods in keying out your background.
NOTE: KeyLight is offered in After Effect to key out more effectively the background.
Resources:
Practice Green Screen
http://www.hollywoodcamerawork.us/greenscreenplates.html
View a sample Clip of placing Trackers in you Green Screen scene
http://www.hollywoodcamerawork.us/vfx_sampleclips.html
Learn more about KeyLight
http://www.thefoundry.co.uk/products/keylight/
Learn about Sky replacement
http://www.videocopilot.net/tutorials/advanced_sky_replacement/
