What are the differences between an Ubuntu Long Term Support release (LTS) and a normal release?
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 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.
The most important thing (for most people) is how long you get to use an install without having to do a release upgrade. A non-LTS version of Ubuntu only gets updates for 9 months from its release so to stay up-to-date —which is critically important— you need to upgrade twice a year; you need to upgrade through every Ubuntu version…
Conversely an Ubuntu LTS release is supported for 5 years and you can upgrade directly from LTS to LTS. This gives you long-lived, solid base to target and test on that makes it super-easy to release-upgrade when you decide to. It's therefore ideal for mass deployment, high-availability systems, and just people who don't like doing release-upgrades.
In the last two LTS versions, point-updates have also been made available to support newer hardware (it's a kernel, driver and X stack), which boosts the utility of the LTS versions over their lifespan. The original stacks are maintained too.
Most other applications won't jump versions, so it makes for a solid, predictable deployment.
In terms of the Ubuntu development process, Ubuntu pulls many of its packages from Debian. Debian has multiple versions too (stable, testing and unstable) that usually correlate with package age. The package pull for an LTS will favour the more stable Debian version. I'm certain there are exceptions to this.
There also is supposedly more focus on bug fixing for an LTS release. More people have a vested interest in the success of an LTS so I'd like to think more people test it pre-release.