Chip Bennett is currently in the middle of creating an updated version of the template hierarchy diagram which shows the structure of how a page is displayed. The new diagram is a bit more detailed than the old one. These diagrams are an excellent way to learn about Template Tags in WordPress as well as visually see the process for determining which page is displayed.
Chip Bennett is looking for feedback on his version of the template diagram. You can either comment on this article with your feedback or give him a shout out on twitter via @Chip_Bennett.