What are .onion domains
On top of providing encrypted and anonymous access to the Internet, Tor also provides feature publicly known as Deep web or Dark web. Unlike regular websites, these websites are not hosted on publicly known IPs. The domain system is independent of any DNS servers, it uses advanced asymmetric and hashing cryptography to ensure security through .onion domains.
An example of such address is http://astrillrcpusdryujyzbsl7wsugvyfc5s6veyrpkojiqbrorgkfdxvid.onion/ - Astrill VPN official Deep web port. The domain itself serves as a key in decentralized database to locate desired website and securely connect you while ensuring that no nodes in the chain know about neither who is hosting the website nor who is accessing it.
Note that firefox needs a special configuration step before .onion domains can be opened. All other browsers work out of the box at this time.