Chroma Keying
Chroma keying is a simple concept but unkown to most beginners. Chroma keying is just using a solid background color when doing any filming so that the background can be removed(made transparent) and replaced with a different scene/film/background. Usually pure blue or green is used for the background because those colors, in its pure form, are not naturally occuring in human skin. If you are doing claymation and you have those colors in objects and characters in the scene any solid color background would do. Just remember though lighting is the key with any color you use. Check out these pictures for a second too. Notice how the green and blue screen in the background is kept free of shadows. Also having a real life actor cover himself in the chroma color allows you to replace him with a computer, clay, or hand drawn character.
With the newer video editing programs(Adobe Premiere and Ulead's Media Studio Pro are most notable for home user) chroma keying is done very easily.
Just select the clip with the solid background you want to be on top and put it on the higher ranking track. Click on it and somewhere in the menu should be an option to make a certain color transparent, you might have to go into filters or something first. If you are using a blue or green screen look for green/blue screen option, if you aren't using either of those colors selecting chroma or key color and select the background color as the transparent color. If there is some shadowing or the actor is wearing a similiar color use the options to select color similiarity until it looks good. Now put the background scene one track lower and that is it.