Categories vs. tags
From Georgetown University Commons Knowledge Base
Categories are a "top down" approach to organization where you can create a category, for example, Chapter 1, and then file posts under that category. Tags embody the opposite approach; after writing a post you can assign to it descriptive words in a stream of consciousness manner that illustrate the post's content. In short, categories and tags help you organize your posts in a way that will make retrieval of information as efficient as possible.