When WordPress 2.8 came out, we all discover the wonders of taxonomies, and how we could add extra information to our posts and custom post types really easily.
However, it turns out taxonomies have existed for a long time and are not just available for posts, but for all object types. I for one had no idea about this!
Luckily, Justin Tadlock is here! In this tutorial, he covers the basics and examples of what you can do with custom user taxonomies in WordPress; a must-read, as with all tutorials from Justin.
Read the tutorial here: Custom user taxonomies in WordPress.