Search Engine Journal: Exposing the Invisible Web to Search Engines

http://www.searchenginejournal.com/exposing-the-invisible-web-to-search-engines/4771/
"The Invisible Web (aka Deep Web) is that humoungous slice of the Internet’s web pages that traditional search engines either have not indexed or cannot index. Often, if they cannot index a page, it’s because the page is database-driven and requires a human trigger before it is rendered in your web browser. For example, you may have to ask a question, such as “show me all the job listings for project manager,” using an HTML form to enter information. In other instances, access to a web page requires authorization such as username and password."