Computer Operating Systems

I'm sure some of us may not even know we have a choice of Computer Operating Systems, other than Mac or PC.  Some of us may have herd of options like Linux, but have no idea of the many FREE Computer Operating Systems they can have.
You could even get a choice at computer start up.  This area is intended provide information I have gathered over the years about Linux and it's subsystems.

(Example:  Shown here is  an older version of Linux Ubuntu that I am running on the laptop I am now using to write this)