The Internet is not just a data storing system, it is also for communication and data transmission between certain terminals. The Deep Web is another network where people use their own servers to keep it more private and closed. According to their levels of accessibility, the Deep Web is separated into these levels;
Level 1 (Surface web)
These are the websites you can access without using a special browser. Although the websites you reach every day can be considered as Level 0, Level 1 websites such as Reddit and Vampire Freaks are considered as a gateway to the dark web.
Level 2 (Bergie web)
They are the websites you can't reach with search engines. FTP servers, +18 contents that are not indexed at this level. It's impossible to find them on search engines because they are either blocked or they are at the very last page of search results.
A proxy may be necessary for the rest of the levels.
Level 3 (Deep web)
Level 3 is split into two groups. The first group is the websites reachable with a proxy, they don't have the .onion extension but they are still banned from the search engines as they have illegal content. Illegal research reports, hacked materials, assassination videos, disturbing adult content, celebrity gossips, and computer security related subjects happen at this level.
The real deep web starts here. Web sites are on private servers and have .onion domains. They are accessible by using a directory, and the contents will most likely contain:
Hacked server data, data dealers, confidential government documents, WikiLeaks archives, international espionage, guides on how to make weapons and bombs, information on terrorist groups, federal intelligence, illegal experiments and scientists, shell network, assembly programmers, hardcore hackers, Microsoft network security information, data analysts etc...
These websites are generally on private servers with HTML codes. Creators can do anything to protect their websites...
Level 4 (Charter web)
Level 4 is also analyzed in two groups. In the first level where you can enter using Tor browser, there are addresses you can't reach with directories like hidden wiki. The contents are claimed to be as the following... (we say claimed because deep web is full of urban legends and we don't have actual proof of the contents of any levels..)
Insider trading and secret company information, prearranged million dollar bet information, world famous arms dealers, tank, missile and defense systems sales, banned movies, videos, books, music, high ranked government spies, important voice recordings, mercenaries, private military, special task forces, black boxes, illegal and banned adult content, sex tapes of government officials, detailed secret wiki encyclopedia, human trafficking, sales of banned chemicals and drugs, organ and blood trafficking, child porn, cannibals and vampires looking for people to murder, the search for illegal experiment test subjects...
Theoretically, Tor browser is sufficient to reach the internet levels mentioned above...