On NETTUTS.com there's a new tutorial that talks about a feature of the upcoming WordPress release (3.0) - taxonomies - and how to create custom ones for your installation.
WordPress 3 fills in a number of important gaps towards being a serious content management system. The easy-to-use custom taxonomies function gives site designers some powerful tools for building a good information architecture. Learn what taxonomies are, why they’re useful, and how to use them in today’s tutorial!
Taxonomies are ways to classify things and can be considered attributes about an object. In the case of WordPress, the only taxonomy that's been available has been the "Category" selection for each post. With this new functionality, it gives you even greater control over the classification of your posts/content. The tutorial shows you how to create simple, nested and hierarchical custom taxonomies and how to use them in your application's code.