The System Metaphor is one of the less popular parts of Extreme Programming (XP). Newer descriptions often even omit it. But metaphor is useful – when you have a good one, it really helps you understand and organize your system.
This post has a couple versions of a tutorial that Steve Wake and I developed and presented at some of the early Agile conferences. It's released under Creative Commons CC-BY-SA.
Thee's also a SlideShare version.