Movable Type has both tags and categories, and typically, I have been recommending that users implement which one feels more comfortable.
But I just saw a comment which gives me a new insight:
I like the analogy that Elise over at SimplyRecipes.com gave me: categories are chapters in a book, tags are the index at the end — Jesse Gardner (Comment)
That is, categories give a viewer a global idea of what topics are covered, but tags can be more fine grained. I’m not saying I’m going to be tagging all my old entries, but it’s good food for thought.