most commonly used Linux version?

[Jonathan Hutchins]

RedHat is probably the most recognized and specified distro in the business 
community (ie by managers as opposed to IT people.  A lot of the 
documentation at http://tldp.org was based on (Pre-Enterprize) RedHat, and 
the book Running Linux, and excellent introductory reference, was based on 
it.  Running CentOS, the recompiled open version, or Fedora, the "community" 
based free version is a good way to learn the Red Hat Way.

Debian, on the other hand, is probably the most used Server OS, excluding the 
above enterprize environment.  When the IT staff have chosen the distro 
instead of Marketing or Management, the server run Debian.  It's 
upgradability, reliability, and long-term stability are second to none.

A lot of the popular distributions are re-workings of Debian that use newer 
packages.  Ubuntu, Mint, and Arch are all based on Debian, as are many 
others.  Mandriva is the main Red Hat based distribution that's not 
affiliated with Red Hat.  SuSE uses the same package system as Red Hat, but 
is very different.  Gentoo and Slackware represent their own branches of the 
tree, with Slackware being one of the oldest.

Ubuntu is the populist.  SuSE is the Novell of Linux, IBM bases it's POS 
systems on it.  Gentoo was the most popular a few years ago, but is a hobby 
unto itself.