What are the differences between an Ubuntu Long Term Support release (LTS) and a normal release?
There is a new release every 6 months (apart from Dapper, which was delayed 2 months). Every two years, the release is a Long Term Support version.
Now, support means:
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
As you can see from the diagram, people who have installed 12.04 LTS don't need to worry about replacing it until 2017! Fantastic. :-)
As if this wasn't enough, Ubuntu releases additional versions of the last LTS between releases - such as 12.04.1, that incorporates 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:
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.
Previous desktop LTS releases were supported for 3 years.
However, Ubuntu 12.04 LTS will be supported for 5 years both for desktop and server releases.
Canonical provides security updates for the LTS releases for 3 years on the desktop and 5 years for the server version. The normal release is only supported with updates for 18 months.
Simply put, LTS releases introduce fewer new technologies than Normal releases, and replaces them with Long Term Support of the older, more time-tested technologies that have proven track records of working like said.