Hello!
I work in Cyber Security :)
There are three areas of the 'Internet':
surface web - anything that's searchable with a direct result from google/browsers and you don't need to log in for
Deep web - stuff that you need to log in to view or have the correct permissions (think 'friends only' view of your Facebook profile)
Dark Web - the place where all your credit card information, email addresses, passwords etc get sold if your account gets compromised. Drug trafficking, human trafficking, illegal porn etc etc.
The Surface web makes up around 4% of the entire Internet. Deep web is circa 90% (think medical records, bank accounts, anything you need to log in for). The dark web makes up about 6% so it's actually pretty huge.
ToR stands for The Onion Router, which basically means you can browse the web in a way that it's super difficult to determine your actual location. I could sign in from America and the traffic hitting these websites would look like its coming from Japan. Very difficult to trace, hence so much illegal activity!
Best bit of advice I will ever give anyone... DON'T USE THE SAME PASSWORD FOR ANY OF YOUR ACCOUNTS. If one of those websites is compromised your email and password combination will be on the dark web quicker than you know it. To check your email address for known compromises head to www.haveibeenpwned.com 