The onion router, or Tor, is a free, open-source network run by volunteers that allow the user to anonymously browse the Internet. Tor network is based on nodes or relays, which are servers run by volunteers through which the user's data is routed. When data travels through one of these relays, a part of its encryption gets removed (or peeled like with onion) to reveal the next relay. Finally, when the final relay is reached (the exit node), the last bit of encryption is removed.
Because of this, it's only possible to see the previous and next relay or node, but not the final one.
To access the Tor network, the user must first download and install the Tor browser on their computer from the Tor Project. The Tor browser is available on Windows, OS X, Linux, and Android.
There are special services that can detect Tor users.