Lexonomy has a great primer on taxonomies for beginners like me…using tags or metadata for navigation/browsing purposes and/or for search purposes, …also has some great links.
Explains first step in a controlled vocabularly is synonym contol:
If you want to control the vocabularly even more then arrange the terms in a hierarchy: taxonomy
Finally, make it more sophisticated than a directory by establishing relationships between all these terms: thesaurus…which is a type of taxonomy.
One day I’d like to get my head around all these terms and how they are related vertically and horizontally:
Classfication (faceted, etc…)
Controlled Vocabulary (post, and pre-cordinate)