The dark web is the least-accessible part of the Internet
Broadly speaking, the Internet can be divided into 3 pieces. The most visible is the "public web," also known as the surface web, which is composed of public-facing websites that search engines can find. This includes social media sites, news outlets and blogs, online stores, community forums, and other sites that anyone can access freely and easily. While most people spend most of their time on the public web, it only accounts for 4% of the entire Internet.
The vast majority of the Internet—more than 90%—belongs to the second piece, the "deep web." These sites aren't accessible to the general public: They aren't indexed by search engines and don't show up in search results, and they may require login credentials or charge a fee for access. That's not necessarily because they're engaging in illegal activity. Common examples of deep web sites include internal corporate sites, university intranets, and online databases.
The dark web is the smallest and least-accessible of the Internet's 3 parts. It's not a single place, but rather a collection of websites. Like sites on the deep web, they aren't indexed by search engines, but on top of that they also generally require special software to keep the websites and their users anonymous.