“Dark web” which is not to be confused with “Deep web” is that part of the web which cannot be accessed by traditional search engines likes Google, Bing or Yahoo. In addition to this, it can only be accessed by special browsers like ‘Tor – the Onion router’ or ‘I2P'(Invisible Internet project). The “Tor router” enables anonymized browsing of the “dark web”.
“Dark web” domains end with “.onion” and are purposefully hidden from popular search engines. They are used to host a number of illegal activities.
Images source: Google images