Free, open-source operating system based on Debian GNU/Linux. Ubuntu questions are on topic here, but if you prefer an Ubuntu-only audience there is AskUbuntu.com.
The related Stack Exchange site, AskUbuntu, exists if you would prefer an Ubuntu-only audience, as opposed to the multi-environment/operating system one found here on Super User.
The Development Version
The development version of Ubuntu is not recommended for beginners because it often contains major bugs and incomplete functionality. The release currently in development is ubuntu-15.04 (Vivid Vervet). Alpha, Beta, and Release Candidates of each development version are released for testing purposes starting about two months (one-third) of the way into the development cycle, as defined by the Release Schedule.
The release schedule of the current development version, Utopic Unicorn, can be found here.
At any given time, two or three Long Term Support (LTS) versions are supported as well as the most recent prior non-LTS version. If the latest stable release is non-LTS, then two non-LTS releases are supported: the current and the prior one. If the latest stable release is an LTS, then only the prior non-LTS release and the LTS releases are supported.
Support means that security and bug fix updates are continuing to be released to the update repositories. When support is dropped for an old release, responsibility for fixing any bugs or security issues is left up to the user.
The released, currently supported desktop versions are:
- ubuntu-14.04 (Trusty Tahr) Current LTS Version
- ubuntu-13.10 (Saucy Salamander) Current Stable Version
- ubuntu-12.10 (Quantal Quetzal) Prior Non-LTS Stable Version
- ubuntu-12.04 (Precise Pangolin) Prior LTS Version