Sailfish OS is a Linux-based mobile operating system developed by Jolla in cooperation with the Mer project and supported by the Sailfish Alliance.
Mer is a mobile-oriented software distribution, targeted at hardware vendors to serve as a base for Linux-based operating systems. It is an important element of the Sailfish OS.
MeeGo was a Linux-based free mobile operating system project, which lives on in a fork called Mer.
Jolla is a Finland-based company that develops and sells smartphones and the operating system Sailfish OS, which powers them.