What is the deep web (or invisible web or hidden web)?
The deep web, also known as the “hidden Internet or “invisible web” is that part of the Web which is inaccessible to search engines. It is not so much invisible as harder to find, since it is often in a different format (PDF, Word file, graphic) or location (specialized, subject-specific or subscription databases) than other web pages. Search engines like Yahoo! and Google now include some of these sources in their results.
