The Dark Web is a term that refers 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, they are called hidden services.
Almost all sites on the 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. When a website is run through Tor it has pretty much the same effect.
Thus, you won't know exactly what system you're accessing unless they tell you, and they won't know who you are unless they do - or unless one of you is careless. The sites on the Dark Web can be visited by anyone but it is very hard to know who is who.
Some examples of Dark Web sites include the Silk Road market (now defunct). Silk Road was a website for buying drugs or any sort of illegal goods that could only exist thanks to the anonymity provided by the Tor network, otherwise it would be instantly seized by the authorities. But there are also legitimate uses for the Dark Web, people living in closed, totalitarian countries are able to skip the censorship thanks to the Tor network. This is not a matter that should be taken lightly given the recent information about US and UK governments massively spying on their citizens.
How to access the dark web?