For the people who are genuinely confused, Jolla is a company that sells smartphones, their first smartphone is also called Jolla; it runs Sailfish OS, which is effectively a fork of the Meego operating system which Nokia used in its N9/N900 phones, before they switched to Windows Mobile. This is not very surprising, because Jolla is a Finnish company principally made up of ex-Nokia engineers, including a lot of the people who created the N9 in the first place. Sailfish OS stays closer to Linux than Android, using Wayland, Qt/QML, PulseAudio etc, and adhering to the Linux Standard Base specification. It also comes packaged with an Android runtime based on Alien Dalvik, so that it can run Android applications (similar to Blackberry 10), although it won't have access to Google Play or the Play Services APIs. The phone is being launched this week in Finland, and there are plans for release in the rest of Europe, and in China, though not so far in the US.
Edit: Some more technical details about the OS: https://sailfishos.org/about-technology.html