The visible web is what you see in the results pages from general web search engines.Theweb spider easily index the page it has crawled.
The invisible web is the hidden area of the Internet that is not searched automatically by search engines. This information is not retrieved by normal search requests.The invisible web is sometimes referred to as the Deep web .
It is said that invisible, or deep web as being 500 times bigger than the searchable, or surface, Web.
There are some web pages that are on the invisible web purely for technical reasons.
Invisible web contains
1) Web pages that needs a password access.
2) Any link that has a question mark ie script-based pages, whose links contain a ?.
3) If a web page has been created without the usual HTML such as pdf, Word, Excel, Corell suite, etc then it may not always be easily read. For instance, a PDF document might only be accessed as a link and not easily found as an HTML web page so such pages also comes under invisible web.
All licensed article, magazine, reference, news archives, and other research resources that libraries and some industries buy for those authorized to use them comes under invisible web content.The contents of these are not freely available: libraries and corporations buy the rights for their authorized users to view the contents.
A way of accessing material on the invisible web is simply to put the word database after your search query..
plane crash database
toxic chemicals database.