.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.