We just learned about the Mac OS.
Another two Operating Systems are called Unix and Linux.
Unix was created in 1970, and Linux was created in 1991 to be a lot like Unix but free.
Unix and Linux are very powerful systems, and unlike Windows and Mac OS you can use a whole bunch of different programs that act like windows for the things you want to do.
Things like Gnome, KDE or X11 are all programs that are separate from Unix and Linux, and you can pick which one you like best based on how they look.
(from: wikipedia - linux)
Kid Facts - Blast from the past: Valvetrain