Not Everyone Knows Dark from Deep
Andy Greenberg writing at Hacker Lexicon, the Wired explainer series, notes that even the well regarded news program 60 Minutes is confused, having described the Dark Web incorrectly as a "vast, secret, cyber underworld” that accounts for "90% of the Internet."
Greenberg says the Dark Web isn't particularly vast, it's not 90% percent of the Internet, and it's not particularly secret. It's a collection of websites that are publicly visible, yet hide the IP addresses of the servers that run them. So anyone can visit a Dark Web site, but it can be difficult to figure out where they're hosted—or by whom. Most Dark Web sites use the anonymity software Tor, though some use a similar tool called I2P. Both of those systems encrypt web traffic in layers and bounce it through randomly-chosen computers around the world so it's hard to match origin and destination of web traffic. Online marketplaces such as Silk Road2, the illegal drug site recently shut down by federal investigators, and other black market sites selling contraband, are usually part of the Dark Web. Wikileaks created a dark site to accept anonymous leaks.
The Deep Web is a collection of all sites on the web that aren't reachable by a search engine. Those unindexed sites do include some in the Dark Web, but they also include much more mundane content like registration-required web forums and dynamically-created pages like your Gmail account.