How do we store information? On a physical library we store books on shelves, and index their position in catalogues. Since the beginning of the web we started building websites like we were used to build libraries, and at the beginning search engines were catalogs of websites.

But digital information is different: it allows content to be reorganized dynamically, and we don't need categories anymore, because we already have heaps of keywords in each document, that can be searched.

I just found (thanks to Daniela) via information aesthetics, a cool video that explain in a very easy way why. Made by the same person that made the Web2.0 for dummies video this spring:

And another document that is very interesting to read about the topic is: Ontology is Overrated: Categories, Links, and Tags