There's a difference between the "Deep Web" and "Dark Web." While the "Clear Web" is the surface area which is indexed by search engines such as Google and Yahoo, the Deep Web is an area search engines can't crawl for or index. Plunging in further, the Dark Web is a small area within the Deep Web which is intentionally hidden from discovery.
You can't use standard access methods to gain entry into the Dark Web. The most common method is through the Tor network, an anonymous network created from nodes which disguise online activity. In order to use Tor, you need the Tor browser, and may also need to be issued an invitation to access certain .onion domains hidden within the Dark Web.
An .onion address is the result of Onion networking -- low-latency communication designed to resist traffic analysis and surveillance. The use of Onion networking is not a perfect solution to maintain anonymity, but it does help disguise who is communicating with whom.
Many of us heard when the underground marketplace Silk Road, one of the largest hidden within the Tor network, was taken down following an investigation by US authorities. However, there are many more vendors peddling their wares within the Dark Web. While drugs are the most commonly-thought of when it comes to the secretive area, you can also purchase a plethora of other illegal goods. Weapons, porn, counterfeit money and fake identities, hacked accounts and even hitmen can be found if you have the cash. If someone annoys you, sending over a SWAT team as a "prank" is also possible.
A quick browse and I could buy lifetime membership passes to popular services such as Netflix, old consoles, clothing, emulators and DVDs, a car or two and bulk weight loss pills. Technology is also popular -- there is a wealth of devices available -- both counterfeit and apparently legitimate -- if you know where to look.
So-called "ethical" hacking and political forums, archives of forbidden books, tips on how to care for your cat -- there are potentially thousands of private .onion addresses hosted which go beyond marketplaces.
Whether you take a risk with buying bargain designer clothes on the Clear Web or sink a few Bitcoins in purchasing illegal items through the Dark Web, neither is risk-free.
Vendors and sellers might be trying to avoid the eyes of legal enforcement in the darker side of the Internet, but this doesn't stop scams from taking place. Scam vendors and quick grab-and-run schemes run rampant -- especially as there is no way to follow up with failed sales down the legal route.
How do you trade without being linked to bank accounts? Virtual currency is the most common method, which includes "tumbling," a laundering process which destroys the connection between a Bitcoin address which sends virtual currency and the recipient in the hopes of covering a user's tracks. Some vendors offer escrow services which holds Bitcoin in trust until goods have been delivered and both parties are happy -- although value fluctuations linked to Bitcoin use makes this move risky.
Aside from using the Tor browser and VPNs, a number of buyers and sellers use "Tails," free software which can be booted from flash storage to provide end-to-end encryption for your browsing sessions.
To further cover their tracks, vendors and sellers will often also use public Wi-Fi hotspots to conduct their business.
Although far from exhaustive, the best Clear Web resource to bounce around and learn a little about the darker, nastier aspects of the Internet is on Reddit. There are sub-forums in which Dark Web vendors and buyers exchange news, thoughts and seller reviews. Advice is also issued on how best to "clean house," create safe "drop" zones to pick up packages ordered from the Dark Web and what to do if you think law enforcement is keeping an eye on you.