I've read that Ubuntu (ie 15.10) takes its code from Debian unstable. So where does the code for Ubuntu LTS come from? Is it also from unstable or more from the testing branch?
I'd like to move from Debian to Ubuntu server with LTS so I don't have to do yearly upgrades and instead hold off for 2-3 years. I know Debian 8 now offers LTS, but a few of the packages can be a bit behind.
It's true technically that Ubuntu LTS is based on a snapshot of Debian Testing whereas other Ubuntu releases are based on Debian Unstable.
However, this is a simplistic way of looking at it. It mustn't be forgotten that Debian Stable, too, comes originally from Debian Unstable, and Debian Stable is a very polished final product - so the fact that something spent time in Debian Unstable earlier in its life cycle is not an indication of its quality in the final release. After Ubuntu branches off the packages it imports from Debian, Ubuntu and Debian then independently continue the process of fixing and tracking bugs and getting packages ready for release, following their own separate processes and timelines.
In the case of Ubuntu LTS, it branches off from Debian at a later stage in Debian's process: once Debian's branched it off into Debian Testing - implying Ubuntu will benefit from more of Debian's own testing and bug fixing prior to the point at which Ubuntu's process starts. The primary way in which Ubuntu will benefit is reduced effort getting the packages ready for release. Ubuntu has different release goals and requirements to Debian which indicates that an Ubuntu LTS won't necessarily be equivalent to a Debian Stable.
Another thing to consider is the distinction Ubuntu makes between packages it supports officially (eg main/restricted) and packages it provides only "community support" for (eg universe/multiverse). The packages in the latter will be modified relatively little after the import from Debian.
And lastly, there are quite a few packages in Ubuntu that are not sourced from Debian at all, for commercial or licensing reasons or because they are Ubuntu specific.
First off the idea that each Ubuntu release is built on a snapshot of Debian is fundamentally incorrect. Ubuntu releases are based first and foremost on the previous Ubuntu release.
During each Ubuntu release cycle there is a period where packages that were not modified by Ubuntu and where the version in Debian is newer than the version In Ubuntu are automatically imported from Debian.
Nowadays for both regular and lts releases Packages are imported from Debian unstable into the "proposed pocket" for the new Ubuntu release. After building successfully and passing some other consistency checks those packages then migrate to the "release pocket".
Some (possiblly all, I'm not 100% sure) older LTS releases made before the "proposed pocket" was introduced did import from testing instead to reduce the chance of importing brokenness.
LTS takes it code from Debian Testing. If you are running a server I would stick with Ubuntu 14.04 LTS until 16.04 LTS comes out. I probably won't switch to 16.04 LTS when it first comes out on my server.
I switched to ubuntu because of issues finding the latest packages with debian. How Ubuntu uses PPAs is awesome to say the least.
