.onion
.onion is a pseudo-top-level domain host suffix (similar in concept to such endings as .bitnet and .uucp used in earlier times) designating an anonymous hidden service reachable via the Tor network. Such addresses are not actual DNS names, and the .onion TLD is not in the Internet DNS root, but with the appropriate proxy software installed, Internet programs such as Web browsers can access sites with .onion addresses by sending the request through the network of Tor servers. The purpose of using such a system is to make both the information provider and the person accessing the information more difficult to trace, whether by one another, by an intermediate network host, or by an outsider.
The .onions are used to stumble services hosted as HiddenServices by the TOR Network. They are randomly created. Further you can bruteforce for starting-letters of a .onion domain by using various services available for Windows and Linux/Unix/BSD.
How-To create
An .onion address is created automatically by the key you receive from the TorBrowser bundle. Please refer to the Tor project for more details on how to obtain such an address.