Debian

Update: http://dreamhost.com/dreamscape/2013/06/03/change-is-in-the-air-dreamhost-upgrades/ 

Debian Linux
DreamHost's servers run Debian, one of the Linux distributions most commonly used for production servers. Debian is known for its stability and security, as well as for Advanced Packaging Tool, an extremely powerful and easy to use package management system. It is noncommercial, and maintained entirely by volunteers.

Debian is also known for its long release schedule. The current "stable" distribution of Debian GNU/Linux is version 6.0, codenamed "squeeze" which was released on February 6, 2011. The current latest stable release is 6.0.5, which was released on May 12, 2012. Most Dreamhost servers are running version 6.0 ("squeeze"), with some older servers running version 5.0 ("lenny"). New Debian versions are released, in one of the maintainer's words, "when it's ready" and not a day sooner. Luckily, the release procedure is transparent, so that any given time, one can download the upcoming release, which is labeled as "testing". Brand-new, untested packages are also sometimes released in the "sid" distribution, which is labeled as "unstable". The current version is labeled "stable". Despite the labels, one should not assume that anything other than the current "stable" version is actually unstable and therefore unfit for use. There are thousands of people using the not-yet-official versions with little or no problems. Debian's reputation for stability is well-earned.

According to Distrowatch.com, a good site to follow Linux Distributions, there are over a hundred active distributions based on Debian. One of the most popular is Ubuntu by Canonical.

Keeping Debian Current
Keeping Debian current appears particularly important on Dreamhost as, as for not all but some notable 3rd-Party Software, as not for WordPress but for Subversion, the version of a program installed & supported on Dreamhost seems to be not its current release but (typically prior release) which has been already tested & packaged by packages.Debian.Org and by backports-master.Debian.Org for the particular version of Debian running in your Dreamhost account (for example, only to Debian packages of Subversion including backports), so if you want such 3rd-Party Software updated, you will need to ask/encourage Dreamhost to update the Debian version you're running (at least in my experience for Subversion, CaydenMC).

To find out the latest versions of Debian available
See Debian Releases on Wikipedia for a clear & typically-very-current history summary. You can also see Debian.Org/releases for the official answer but currently that seems not as well-organized including no history table.

To find out what version of Debian your Dreamhost account is running

 * 1) Connect to your Dreamhost account via a secure shell, as via PuTTY
 * 2) Per the official Debian FAQ entry on how to tell which version is running, at the shell prompt, type "cat /etc/debian_version" and only the currently-running version number should be output.
 * For example, "5.0.8" is what my account is running right now, but version 6 was released 6mo ago and is labeled "stable" so I'm planning to ask Dreamhost update my account --CaydenMC 20:14, 21 August 2011 (PDT)

To request your Dreamhost account be updated to a particular version of Debian

 * This is supposed to happen automatically but doesn't happen immediately, so you you may want to request it if you are in need of particular later version including if you having apparent-bugs which you think might be fixed by a newer version.
 * Note that Dreamhost does not install Debian releases which are not marked as stable.
 * At least for normal hosting accounts, this requires transferring your account to another computer which is already updated to the newer OS, and sometimes means some updating-links & DNS on your end, as described & documented at Server Moves.
 * To request this, contact Dreamhost support.