I still don't agree with that newfangled "dark" terminology.
Anyway Tor is not the only one, but all the posts here seem to focus in Tor, maybe some would think that's "The" Dark web? Wrong.
Its not union its onion. Tor means The Onion Router.
You can publish content that is only available inside Tor, in their terminology its a "Hidden service". The best thing is you can publish it from your PC and no one needs to know its you, so its great for freedom of speech. When you do this, it creates a .onion address that is only visible when using Tor.
Because its a separate network, it has no relation with the "normal" web, this also means all the official domain servers for internet do not exist inside tor, so no .com for you. There is only .onion, and nothing else. Its more like a convenience to tell browsers (and users) you want something not from the normal internet, but from tor.
Because there is no control in what people can publish, it is possible to find "questionable" content in there. Doesn't mean its the majority or even the main reason it exists. Tor is an invaluable tool to defeat censorship, but yes it was actually made for Navy agents to pass info from behind enemy lines. Its is nice of them to share their project to the world, since its open source you can be sure there are no hidden backdoors. (Same way the NSA gave us SELinux).
I hope you don't lose your excitement expecting to see some sort of matrix like thing when it could actually be some boring text page about things you already know or even someones blog using a pseudonym.
It is said Satoshi used tor sometimes. The confusion may arise because tor also allows access back to the "normal" web, but this access comes from volunteers willing to run exit nodes. Unfortunately all sorts of people abuse this and very often those IPs are flagged even if you have done nothing wrong there was some idiot script kiddie insulting people in some forum or trying "hacking tools" against others from the same IP.
There are some projects that don't have this link back to the normal internet, bypassing the problem but becoming relegated to obscurity. Maybe that's why they call them "Dark"? It still sounds silly to me.
Anyway i refuse to call Tor deep, dark or whatever sounds cool. Its just another tool to defeat censorship. It kinda resembles the Internet in the 90ies, before "normies" invaded it.