ITworld.com has an interview with David Weinberger:
In the past few months and with the pace picking up, there’s been a huge amount of development in the realm of tagging online and digitally. The differences between the sort of real-world way of organizing stuff, which in its finest incarnation, you have somebody who builds a universal taxonomy, a set of classifications into which everything can fit, you know, like the Dewey Decimal System. Every book can find a place in this preset set of categories arranged into a hierarchical tree.
The absolute opposite of that is what’s happening on the Web now as more and more sites allow people to tag content, whether it’s bookmarks at del.icio.us or it’s photos at flickr. So you post your photo and you put in a word or two that constitutes a tag. And those tags are then made public, so anybody can find all the photos that have been tagged as Grand Canyon or as humorous or whatever. So that’s a bottom-up taxonomy, a bottom-up set of classifications. There’s not a preset set.