The invisible web, or deep web, is estimated to be 500 times bigger than the searchable web and comprises databases and results of specialty search engines that the popular search engines can't index.
Firefox extension offers an addictive way to search via categories or keywords or happenstance; you train it, with thumbs up or down on sites, so it learns your tastes; I prefer searches on keywords so far, and have had little desire to "network"
You sign up, enter the name of a book, then find out what other folks who liked that book also liked; a tag cloud for the book leads to other books on this topic. Quite addictive, and I just used it to look at the info. around one book.
Clarity regarding controlled vocabularies, taxonomies, thesauri, ontologies, and metamodels. With all the scuttlebut going around about folksonomies and tagging, these are important terms to understand. In the process of tagging, it's pretty noticeable