Rock (OMLx 5.0) is the home and workstation edition published by the OpenMandriva Association. Also for individual users who do not want many updates, be it because they prefer their system to remain the same or because of slow internet connection. Releases are scheduled around once a year.
It would be the most stable and suitable for users that like things to stay as they are and just work. Package upgrades will be limited mostly to bug fixes, and security updates.
Rock repositories consist of a symbolic link to the latest stable version of OpenMandriva Lx. Currently Rock is linked to OMLx 5.0, however when a new OMLx point release will be out Rock will automatically be switched to it.
Any stable version will eventually reach EOL (End Of Life) after the new version is released and there will be no more updates of any kind.
The Slim version of OpenMandriva is for you if you'd like to choose the programs you want to use primarily yourself.
It comes with: the welcome application (OM-Welcome), terminal (Konsole), a file manager (Dolphin), an image viewer (Gwenview), a web browser (Chromium), text editor (Kate, Kwrite), archiving tool (Ark), a graphical program to install packages (Dnfdrake), and of course the well known and feature rich OpenMandriva Control Center.
If you don't wish to have specific software and want a system which has programs for all day to day tasks already installed go with the OMLx 5.0 Plasma full version.