To put it shortly:

* There was Meego (a joined project by Intel and Nokia) which merged Moblin and Maemo projects. It included several subcategories targeted for different form factors (Meego tablet, Meego handset and etc.).

* Nokia deserted the project after MS paid them a couple of billions. Intel deserted shortly afterwards.

* Several Meego developers forked the project in order to preserve the accumulated effort and to continue it further. The fork was only of the part of the original project, not of the whole since Meego was quite big and had parts which started getting obsolete (like netbook). This fork consisted of two parts - Mer (MEego Relaunched) which included most core aspects of Meego, and Nemo, which forked the Meego handset (that previous effort was called Meego CE). Nemo uses Mer as a core architecturally. Mer positions itself as a distribution for vendors who create their mobile oriented OSes (and beyond).

* Sailfish was designed by engineers who left Nokia (after Nokia desertion) and created Jolla company, as well as others who joined this effort. Sailfish uses Mer as a core as well, and also uses various Nemo middleware components.

I.e. Sailfish and Nemo are "vendors" of Mer. Mer is a fork of the part of original Meego project. (Also don't confuse Meego with Harmattan which was developed by Nokia, and branded as Meego while not being Meego strictly speaking).