The Deep Web (also called Deepnet, the invisible Web, dark Web or the hidden Web) refers to World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines.
http://en.wikipedia.org/wiki/Deep_Web
What is the Invisible Web? Is it some kind of Area 52-ish, X-Files deal that only those with stamped numbers on their foreheads can access? Well, not exactly. The term "invisible web" mainly refers to the vast repository of information that search engines and directories don't have direct access to, like databases. Unlike pages on the visible Web (that is, the Web that you can access from search engines and directories), information in databases is generally inaccessible to the software spiders and crawlers that create search engine indexes.
How Big is the Invisible Web?
In a word, it's humungous. Bright Planet estimates the invisible, or deep, web as being 500 times bigger than the searchable, or surface, Web. Considering that Google alone covers around 8 billion pages, that's just mind boggling.
Why Is It Called "The Invisible Web"?
Spiders meander throughout the Web, indexing the addresses of pages they discover. When these software programs run into a page from the Invisible Web, they don't know quite what to do with it. These spiders can record the address, but can't tell you squat about the information the page contains. Why? There's a lot of factors, but mainly they boil down to technical barriers and/or deliberate decisions on the part of the site owner(s) to exclude their pages from search engine spiders. For instance, university library sites that require passwords to access their information will not be included in search engine results, as well as script-based pages that are not easily read by search engine spiders.
Why Is The Invisible Web Important?
Perhaps you think it would be easier to just stick with what you can find with Google or Yahoo. Maybe. However, it's not always easy to find what you're looking for with a search engine, especially if you're looking for something a bit complicated or obscure. Think about the Web as a vast library. You wouldn't expect to just walk in the front door and immediately find information on the history of paper clips lying on the front desk, right? You might have to dig for it. This is where search engines will not necessarily help you, and the Invisible Web will.
Plus, the fact that search engines only search a very small portion of the web make the Invisible Web a very tempting resource. There's a lot more information out there than we could ever imagine.
How Do I Use The Invisible Web?
Fortunately for you and I, there are many other people that have asked themselves the exact same question, and have put together great sites that serve as a launching point into the Invisible Web. Here are some general gateways:
* One of the best ones out there is the Direct Search site put together by Gary Price, a librarian and information research consultant. His page is nicely organized into searchable categories and is updated frequently.
* Another good resource is the Invisible Web Directory , put together by the aforementioned Gary Price and search guru Chris Sherman. This site is a directory of searchable databases, organized by subject.
* The Resource Discovery Network has resources mostly from the United Kingdom, and is extremely well-organized and very searchable.
* The University of California, Riverside maintains InfoMine , an incredible resource that at last count included over 100,000 links and access to hundreds, if not thousands, of databases.
* The Virtual Library is simple and easy to use, with annotated subject links. I especially appreciate the annotations because it helps rule out extraneous search time.
SOURCE
A few youtube videos about the deep web/invisible web:
Searching The Deep Web
The Virtual Private Library and Deep Web
Find People on the Web With Pipl
