Opera is a closed source chromium based browser (as of version 15), developed by the company Opera Software.
When version 15 was released 2013, Opera switched engine from Presto to Blink. They also started from the beginning, so many of the features that Opera <=12 had are gone. (Like mail and built-in RSS)
Opera is divided into three streams: Opera stable, Opera beta (formerly Opera next), and Opera developer. You can read more about them here: http://blogs.opera.com/desktop/streams/
Opera is not in the official repositories, the best way to install it is to download Opera from their website, and install it normally. Then it will add the opera repository for you so it updates automatically. You can read more about that here: http://deb.opera.com/manual.html