As you may know, the “web” runs deeper than that network of hyperlinked pages you’re browsing right now.
Technically, the portion of the web that search engines like Google and Microsoft Bing catalog is called the “surface web” (though most people will think you’re a weirdo if you call it that). Less accessible portions go by other names.
For those who care to draw a distinction, the “deep web” refers to the region outside public view. This includes pages not indexed by standard search engines, such as password-protected sites, or ones tucked behind a paywall. Many people spend just as much time on the deep web as they do on the surface, if not more.