This is an intermediate level WordPress tutorial intended for designers learning to customize a theme. If you have a solid grounding in php coding and WordPress—you just want to understand the code—I recommend a separate tutorial and update (to the same tutorial) by Spencer at Function. You can also find code below, but this article is geared towards understanding custom fields, meta data, and how to create and implement custom write panels. Design and content context will be taken into consideration.
Written for users operating with WordPress 2.9.