Tor’s hidden service protocol is normally called the ‘Dark Web’ in the media (if you’ve seen a URL ending with .onion, that’s a hidden service). The protocol is what enables people to publish web services that are accessible on the Tor network. Perhaps the most famous hidden service was Silk Road before it was shut down. Mainstream publications like ProPublica and even Facebook also run hidden services to protect their users.
The primary purpose of the protocol is to enable anonymous publishing and consumption of content, but the protocol can also be used as another way to expose your content (like ProPublica) or used as a practical tool for getting work done on the Internet. In the specific case of publishing, setting up a Tor hidden service is easier than buying a domain, setting up a server, and configuring a firewall and DNS. After setting up a hidden service, it’s also available to users not using Tor via Tor2web Another benefit of publishing on Tor is that mangaing certificates for HTTPS isn’t necessary.
Hidden services aren’t limited to HTTP and can expose other network services like SSH. This could allow you to setup a remote login to your home network that accessible anywhere you can get Tor up and running. Normally to do that you would need to setup dynamic DNS or would need to pay for a static IP. Having that service on Tor provides another layer of security that you wouldn’t have if your SSH service was directly on the Internet. The hidden service can also be configured to only allow certain clients to connect to the service providing yet another layer of security for your service. This kind of functionality is similar to the ngrok software, but uses the Tor infrastructure.
The documentation for setting up a hidden service is well-written, and there is a hidden services best practices document that includes additional information about setting them up. Using the Python Stem library, it’s possible to create hidden services programmatically.
Even if you aren’t an activist in a dangerous locale or Dread Pirate Roberts, Tor’s hidden services are a valuable contribution to the infrastructure of the Internet.