Deep web

The Deep web (also referred to as the deep net or dark web/net) is a hidden portion of the internet that can't be found using generic search engines or requires certain browser modifications and plug-ins to view, such as the Tor browser. Other software and general browsers and their respective modifications exist as well.

Overview
The deep web requires certain plug-ins, permissions, modifications or any other forms of extra ability that would allow them to view generally hidden content. Because of this, a large majority of the internet falls under the category of "deep web", and it is estimated that around 90% (96%-99%) of the internet is part of the deep web.

Some of if not most of the websites visited by the average person may count as the "deep web", such as personal information (ex: bank account information) and hidden pages (ex: website accounts). Any web-page or POV on a page that needs extra authentication or permissions to view will fall under the category of "deep web".

Dark web
The dark web or dark net is a category of the deep web that hosts malicious, often disturbing, and generally illicit content. It is estimated that the dark web is around 400-550 times larger than the surface web

Markets
Darknet markets refer to mostly commercial websites that exist to sell illegal items, such as illicit drugs, illicit pornography, weapons and arms, among other such things. Most of them will only accept bitcoin as payment, due to its level of privacy.

Hacking & Malware
Hacking groups/services and malware (most commonly adware and ransomware) are very present on the dark web, and are defined as any number of people/bots that have the ability to break into another's private data and manipulate it using a client-sided ability.

Illicit pornography
Illegal pornography, more specifically, child pornography, is one of the most popular services and forms of content in the dark web. It is estimated around 80% of the traffic within the dark web is towards sites or files containing child pornography, despite it being difficult to find at first.

Gore & violence
While gore and graphic imagery is not illegal in most countries, it is very easy to find on the dark web. Beheadings, dismemberment, snuff films, torture, and other forms of violent media make up the majority of said content.

Illicit drugs
Drug marketplaces that sell illegal drugs make up a majority of the darknet markets, along with guns and other such violent weapons.

Weapons & arms
Guns, bombs, and other such weapons are a large part of many darknet markets, along with drugs and pornography. Most of these are often taken advantage of by criminals and terrorists.

Terrorism
Terrorist markets, forums, or any other content that could further acts of terrorism are also in the dark web, and often take form of internet forums and social medias that are used by terrorists to create plans with each other.

Activity
The activity and traffics towards these types of websites and content is generally estimated upon and can only be left up to assumptions, but most estimations are expected to be accurate.