If the federal government's trial against Ross Ulbricht, recently convicted of seven drug and conspiracy counts in relation to his founding of the Silk Road, has left anything in its wake besides unfulfilled online orders for illegal drugs, it's probably the greater public's first awareness of the term "Dark Net." But, what is this Dark Net anyway?
The Dark Net is a subset of what is known as the Deep Web. The Deep Web is simply all of the non-indexable, unsearchable pages that Google can't find, are behind some sort of login, or both. The Dark Net, however, are those kind of sites that, shall we say, prefer to remain anonymous. So much so, that you can't access them at all from a standard Web browser, like Internet Explorer, Chrome or Safari. You need a browser called Tor, aka The Onion Router.
Tor was developed by the U.S. Navy and eventually folded into DARPA initiatives. The Tor browser allows users to navigate and travel through the Web (both "regular" sites and special Tor-only sites) anonymously by putting their traffic through a large number of encrypted servers located around the world, each doing a little bit of location and IP address cloaking along the way.
What does that mean? Basically, a users physical IP address is virtually impossible to trace.
So, you use a non-traceable browser to navigate to unindexed websites typically categorized, to buy whatever illegal substance you wish to attain. I say "typically", because it is thought that nearly 3/4ths of sites in this environment deal illegal drugs, homegrown or prescription.
It is also thought that Deep Web and Dark Net pages combined outnumber the standard Web by at least 5 to 1. But, no one really knows how many pages truly exist.
If you're interested in exploring on your own, you can find a number of guides (like this one) on how to navigate and find Dark Net sites. Now, be careful out there.