The surface web is only about 4% of the total, surface web contain google,Wikipedia,bing and all other sites we visit daily. The other 96% belong to deep web.
The "dark web" is the encrypted network that exists between Tor servers and their clients, we can not access them through a normal web browser.
To access dark web, you can use Tor browser.
Dark web contains mostly the illegal stuff, so it is recommended not to access without a good knowledge about it.
Deep web is also hidden, it contain stuff like medical records,academic info,databases,government resources, and other important records. But there may be illegal sites too.....
it is advised not to access these sites without a proper understanding, because there may be sites which may steal your privacy, or damage your computer.
To be safe always turn off java script of the tor browser.