Group items tagged

How do we categorize information for later retrieval.  Although this "game" is fun and addictive and really serves the purpose of improving Google's image search engine, it has learning value.  Playing this game can help us think about the difference between the traditional way of "filing" information in "folders" for later retrieval and the new, web 2.0 way, of "tagging" information.  How are tags different than categories?  What is the power of tagging in terms of access to our own information as well as the learning potentail from the collective mind?