This lesson gives you an intro to blue-screen or green-screen keying. You also learn how to work with different compositions and nesting composition as precomps into other compositions.
The guru lessons are an After Effects tutorial series for beginners. They teach you step by step all the basics you need to get started with After Effects. The tutorials are freely available in English and German here: http://www.mamoworld.com/index.php?option=com_content&view=category&layout=blog&id=42&Itemid=84