The surface Web (also known as the Clearnet, the visible Web or indexable Web) is that portion of the World Wide Web that is indexable by conventional search engines. The part of the Web that is not reachable this way is called the Deep Web. Search engines construct a database of the Web by using programs called spiders or Web crawlers that begin with a list of known Web pages. The spider gets a copy of each page and indexes it, storing useful information that will let the page be quickly retrieved again later. Any hyperlinks to new pages are added to the list of pages to be crawled. Eventually all reachable pages are indexed, unless the spider runs out of time [1] or disk space. The collection of reachable pages defines the Surface Web.
For various reasons (e.g., the Robots Exclusion Standard, links generated by JavaScript and Flash, password-protection) some pages can not be reached by the spider. These 'invisible' pages are referred to as the Deep Web.
A 2005 study [2] queried the Google, MSN, Yahoo!, and Ask Jeeves search engines with search terms from 75 different languages and determined that there were over 11.5 billion web pages in the publicly indexable Web as of January 2005.
As of June 2008, the indexed web contains at least 63 billion pages.[3]
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