YouTube is, in the words of the creators themselves, "an online video streaming service that allows anyone to view and share videos that have been uploaded by [their] members." For the sake of my argument, let us assume that they are telling the truth.
The various movie clips hosted on the site are divided into a number of fairly broad, predefined categories. However, the site also allows the author (or uploader; not all members of youtube own the rights to the material they post) to add a number of "tags," or keywords that do not necessarily conform to any previously defined structure. As far as I can tell, other users cannot add tags to a movie clip, but they are able to rate the quality of the clip from 1 to 5 and can also post comments.
By combining strict categories with free-form tags, YouTube manages to capture the best of two worlds, in a sense. It offers precision and flexibility in allowing the uploaders to pick the words that best describe the content of their (or others') creations, while simultaneously dodging the trap of nonsensical keywords by allowing the alternative route of a more traditional hierarchy. A possible drawback is that users cannot add tags to movie clips posted by others, but I'm guessing that it is a choice they have made to prevent abuse.
Next to each movie clip is a brief list of "related" files, apparently based on tags. Most of the clips linked in this way seem relevant. Indeed, I have spent more than just a few hours hopping between related movie clips on YouTube. Rarely have I been disappointed while doing so.
Another highly interesting function that I have noticed is that each movie clip comes with a record of sites that have linked to it. This offers an effective overview of the clip's popularity in a global context and fits well with the notion of user-created networks, social or otherwise.
http://www.youtube.com/
