What is the Deep Web?

Duringresearch for my recent post about Dark Nets, I came across the term Deep Weband wondered what that meant. 
TheDeep Web is the part of the Internet that is not indexed or catalogued bysearch engines.  In other words, search'bots for services like Bing and Google and Duckduckgo haven't crawled throughthese areas and noted their addresses and locations, so they don't show up in searchengine results.
Apicture of an iceberg is often used to illustrate this idea, with the smaller,above-water section representing the Internet we all use, and the larger, below-watersection as the Deep Web.  It's a usefulimage, but I don't know if it's accurate because no one seems to know how bigthe Deep Web is.  It may be bigger orsmaller than the regular Internet.
Whatdoes the Deep Web contain? While the articles I found all agreed that beingnon-indexed defines Deep Web content, they did not agree on what that contentis.
Forinstance, one article explained that much of the content is dynamic--pages thatare constantly updated and changed like airline flight information or breakingnews stories--while much of the regular Internet is static.  An example of this is a database where youcan only retrieve information with a query. 
Thereare also fee-based sites that you must pay for before being able toaccess.  The article mentionssubscription services used by libraries, but I would guess a lot of pay sitesare for pornography.  The article alsopoints out the dynamic content of social networking sites like Facebook andTwitter, but users must only be friends or followers to access them.
Thisall sounds fairly benign.  In contrast, anotherarticle I found describes a far different Deep Web in which anonymity is king,and criminal transactions for drugs, credit card numbers and child pornographythrive.  The article has a screen shotfrom a popular drug sale site that lists all sorts of illegal drugs for sale invarying amounts from anonymous vendors. The author also wisely warns people that if they go wandering around theDeep Web they may find sickening child pornography.
Thusthe Deep Web may be anything from a business database to an illegal filesharing site.  While I normally supplylinks to my sources, in this case I'm not because some of them provideinstructions that will only tempt people to explore.  The regular Internet is vast, with manyresources available to you, so you need not view the Deep Web as somethingyou're missing out on.  And if what Iread about much of the content is true, you're better off avoiding italtogether.
(The pic of a sewer is from:  benmayfield.wordpress.)