The dark web is a subset of the internet that requires special software to access. The most common dark web is the Tor network, which routes traffic through a series of encrypted relays before delivering it to its destination. The route randomization and encryption mean observers cannot easily determine who is communicating with whom. This property enables hidden services (.onion addresses) that are reachable only through Tor and whose servers' locations cannot be determined by outside observers.
The dark web is not synonymous with illegal activity, though illegal activity exists there alongside legitimate uses. Major news organizations including the New York Times and BBC operate .onion versions of their sites for readers in censored countries. Human rights organizations provide anonymous contact channels through .onion services. Privacy-focused services including secure email and encrypted messaging publish .onion endpoints. The dark web is a tool; what makes it valuable or dangerous depends entirely on how it is used.
The deep web is often confused with the dark web. The deep web is simply any web content not indexed by standard search engines: your bank account pages, private social media posts, subscription content, and corporate intranets. The deep web is enormous and largely mundane. The dark web is the small portion of the deep web specifically accessible only through Tor or similar networks. These terms are frequently conflated in media coverage but describe fundamentally different things.