Compositing tools let you greatly enhance your projects by adding multiple layers of video and images. The principle compositing tools are opacity, the Color and Luminance Keys, mattes, and alpha channels. I covered the first two in this hour, and will go over mattes and alpha channels in Hour 15.
Making keys work well takes some extra effort. Proper backdrop colors, lighting, and keying techniques all come into play. Green/blue screens work best but invariably take some trial and error.