The Dark Web is a term that refers specifically to a collection of websites that exist on an encrypted network and cannot be found by using traditional search engines or visited by using traditional browsers.
Almost all sites on the so-called Dark Web hide their identity using the Tor encryption tool. You may know Tor for its ability to hide your identity and activity. You can use Tor to spoof your location so it appears you're in a different country to where you're really located, making it much like using a VPN service.
When a website is run through Tor it has much the same effect.