Everyone knows what the world wide web (web for short) is and we all know it is incredibly massive containing most of our human knowledge.
There is a part of the internet that we call the Dark web which is not indexed by search engines. This is a part of the web you can only get to if you know exactly where it is. Search engines (like Google) collect their information using a tool called spiders and crawlers. These crawlers read web pages and follow links in the pages to index the known web. There are parts of the web these crawlers can't access or are specifically blocked from accessing (because they are part of restricted or private networks). Some of these un-indexable hidden web pages (in the Dark Web) are normal access controlled sites and other times they belong to organized crime, illegal groups or simply "bad people".
Wikipedia estimates that the web is about 167 terabytes un size whereas it is estimated that the dark web is over 91,000 terabytes.
As in information security professional, I am constantly looking for ways to search and analyse content on the dark web to protect my company and often have to rely on specialized companies that perform this type of specialized analysis.
What kind of bad things can you find on the dark web?
You can find an entire of illegal economy on the dark web such as drug retailers (Silk Road), child pornography (police conducted a major bust recently), guns, explosives, terrorists, sale of personal info for identify thieves, sale of company secrets, etc.
Above you see a sample price list for drug related material from Silk Road.
Above you see a Dark Web classified site for assassinations.
In addition to operating in the shadows, these organizations use hiding technologies like TOR which makes locating these people very difficult.
The issue is that hiding behind the TOR technology is often used by criminals but it is also used by political activists, journalists and other whose lives may be on the line if their location or identity is discovered.
Tor isn't good or bad. It is a tool that can be used for either.
Read about TOR here.
Most of the Dark Web is simply a bunch of boring webpages not indexed by search engines and that can only be accessed by knowing exactly where these sites are and using the proper access mechanism.
But other parts of the Dark Web are more nefarious and a cause for concern for law enforcement, intelligence agencies and large companies.