Ever heard the phrase? It's the 80 percent of the Internet that isn't indexed, so it can't be viewed by normal means, and cannot be searched. They end in a ".onion" domain and have completely randomized URLs. It can only be viewed by programs that are made to view these pages, one of these is TOR. Because of these things, viewing Deep Web is untraceable, though they can monitor who comes out of the Deep Web, but not what they were doing in it. It's completely anonymous, much more so than 4chan.
The Deep Web is also the most fucked up place in the world. Because it's completely untraceable, every single illegal activity you can think of breeds there. Drug and human trafficking, sites where you can pay someone in your area to kill someone you want dead, like a fucked up version of craigslist. Loads and loads of child pornography, sites teaching you on how to kidnap a child or the best way to kill a family and frame the husband as a murder-suicide, and even forums dedicated to kinks related to injecting themselves with HIV blood and giving that blood to unwitting participants.
All those websites, imageboards, and forums are collected in a wiki called "The Hidden Wiki", which is just what it sounds like. I spent a grand total of ten minutes in the Deep Web, and I noped the **** out of there.