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So categories can be tags but tags cannot be categories. Categories are like the huge signs you see on aisles in supermarkets – “Food”, “Hygiene”, “Frozen” etc, they guide you to sections where you can find what you are looking for. Tags are like the labels on the products themselves. Categories organize, hierarchically. Tags need not. Tags provide meta-information, Categories need not. Tags cross-connect, Categories do not. By cross connect, I mean, when you go looking for posts tagged with “Flickr” on technorati, you find posts from various sources, all about Flickr.