There is a new release every 6 months (in April and October), with the version number being year.month (e.g.: 16.04 was released in April 2016). Every two years, the April release is a Long Term Support version.
All normal releases (13.04 and later) are only supported for 9 months.
All LTS releases (12.04 and later) are supported for five years on both the desktop and the server.
Older versions had slightly different support cycles, but they haven't been included as they're all unsupported now. See the Ubuntu Wiki for historical information.
Now, support means:
Updates for potential security problems and bugs (not new versions of software)
Availability of Commercial support contracts from Canonical
Support by Landscape, Canonical's enterprise oriented server management tool set
The Desktop refers to the packages that are in the main and restricted repositories, these are the ones that have the little Ubuntu icon next to them in Synaptic or are marked as Supported in the Software-Centre respectively.
The Server packages are the ones in the "server-ship" and "supported-common" seeds (there's a directory of all of the different seeds available).
This is what this looks like:
![Image depicting](https://assets.ubuntu.com/v1/ac31b45e-release-chart-desktop-big.png)
Image from Ubuntu.com
The primary reason for using an LTS release is that you can depend on it being updated regularly and therefore secure and stable.
As if this wasn't enough, Ubuntu releases additional versions of the last LTS between releases—such as 14.04.1, that incorporate all of the updates up to this point. This is called a Point-Release (or sometimes snapshot). Those are released every quarter to half year, as needed.
In addition to support, there are Development strategies that differentiate an LTS release:
The base of the operating system, Debian, comes in three versions: Stable, Testing and Unstable. Normally, Ubuntu is based on Unstable; the LTS releases are based on Testing. Starting with 14.04 LTS, all new releases will be based on Debian Unstable.
The Development effort for an LTS release in focused on providing a rock solid base, not only for customers who want the LTS release, but also for the next three Ubuntu versions to come.
Thanks to Oli for demystifying that last part, I wasn't quite sure about it.
Yes
Once you upgrade to 12.10, you will no longer be running 12.04, and no longer running an LTS release.
The purpose of the LTS releases if for those who don't necessarily need the absolute most up-to-date packages, and would rather not upgrade every 6 months, but still get critical security updates and bug fixes for longer than the 18 months of support given to a standard (non-LTS) release.
Best Answer
Because we use Firefox 11 instead of ESR and most of the same reasons apply to Thunderbird as Firefox for using the rapid release version. The decision to not use ESR by default in the LTS was surprising to me, so I asked on the mailinglists (you can find the thread here: https://lists.ubuntu.com/archives/ubuntu-desktop/2012-February/003672.html) and Jason Warner from Canonical answered:
Anyway, you'd like to install Thunderbird ESR, you can donwload it here: http://mozilla.org/en-US/thunderbird/organizations/all-esr.html You simply need to extract it into your home folder and add a script called
~/bin/thunderbird
to launch~/thunderbird_folder/thunderbird $@
every time your user calls thunderbirdYou can also see this blog post from Chris Coulson on why the ESR was not selected.