There is so misconception about the dark and deep web. There are two faces of the internet. The one we always see on the search engines directory is the surface internet. All the content is indexed and crawled by web-spiders. The surface internet constitutes very less of the whole internet, around 8-10%. Everything except that comes under the deep web.
Now, the deep web is not something very complex to access or use. In fact, many of us already access the deep web every day without knowing it. Just think of the things that can not be indexed or crawled. Your emails, internet banking, Netflix accounts, the JR. Vip section here on BHW. There are many more things to it as it is 8-9x times larger than surface internet. Many schools and colleges intranet systems. Companies use deep web for their employees-only websites.
But, some criminal minds when started using the deep web to conduct illegal activities, that part of it came to know as The 'Dark Web'. Now, this terminology is not widely agreed upon by many, but it doesn't mean that it is non-existent. I saw someone write here that there is no such thing as the dark web. I just ask you, have you ever heard about Silk Road? It was the first online drug market comprising of tons of drug dealers selling weed, heroines etc. Its average turnaround was $15 million annually which would have increased to $30-$50 million but it was busted.
The websites on dark web do not use normal TLDs, instead, they use the TLD .onion and are accessible only through TOR. Also, all .onion sites are not illegal. Governments also use the dark web to store confidential Infos.