How does the Deep Web differ from the “surface” Web?
Search engines — the primary means for finding information on the “surface” Web — obtain their listings in two ways. Authors may submit their own Web pages for listing, generally a minor contributor to total listings. Or, search engines “crawl” or “spider” documents by following one hypertext link to another. Simply stated, when indexing a given document or page, if the crawler encounters a hypertext link on that page to another document, it records that incidence and schedules that new page for later crawling. Like ripples propagating across a pond, in this manner search engine crawlers are able to extend their indexes further and further from their starting points. Thus, to be discovered, “surface” Web pages must be static and linked to other pages. Traditional search engines cannot “see” or retrieve content in the Deep Web, which by definition is dynamic content served up in real time from a database in response to a direct query.
