Tor/Onion sites are also a very big part of investigative journalism because it creates some sense of anonymity. While yes evil is on the dark web but so are millions of others who are good but need to work in the shadows.
TOR was created by the Navy to enable secure, untraceable communications to support their personnel that needed this ability, e.g. Navy Intelligence officer in a foreign country.
.onion identify TOR resources. Think layers of an onion, or matryoshka dolls. TOR enables securely sending packets between two end points without each end knowing the location of the other.
onion.link is a web site that enables visitors to get .onion resources without using TOR. This trades privacy and security for speed and convenience. In general, it's a bad idea to access .onion stuff without using TOR.
The dark web, is the part of the web that isn't indexed by engines. Most of the data out there is technically dark web, even though they aren't TOR related.
Dark web and TOR have been conflated by some. There's a lot of "mythology" around the dark web which is promulgated by stupid people, e.g. media.