The deep web refers to parts of the World Wide Web whose contents are not indexed by standard web search-engine programs, in contrast to the "surface web," which is accessible to anyone using the Internet. Deep web sites can be accessed by a direct URL or IP address, but may require entering a password or other security information to access actual content. Uses of deep web sites include web mail, online banking, cloud storage, restricted-access social-media pages and profiles, and web forums that require registration for viewing content. It also includes paywalled services such as video on demand and some online magazines and newspapers.

The deep web is often confused with the dark web, which is a subset of the deep web that is intentionally hidden and requires a specific browser, such as Tor, to access. The dark web is not indexed by search engines and is known for being a hotbed of criminal activity, including black markets for stolen credit cards and personal information, firearms, malware, prostitution, sex trafficking, and drugs.

It is important to note that while the deep web and dark web are often used interchangeably, they are not the same thing. The deep web is a reference to any site that cannot be accessed by a traditional search engine, while the dark web is a portion of the deep web that has been hidden intentionally and is inaccessible by standard browsers and methods.