The base of the operating system, Debian, comes in three versions: Stable, Testing and Unstable. Normally, Ubuntu is based on Testing; the LTS releases are based on Stable.
That cannot be true then for 12.04 because Debian Stable (Squeeze) is almost going to be old-stable, and it's obvious that the kernel versions in 12.04, packages, etc. cannot have come from Debian Squeeze.
So then what other Debian Stable is there for Precise to be based on?
That part of that answer is wrong.
From https://wiki.ubuntu.com/LTS:
- We are more conservative in our package merge with Debian, auto-synching with Debian testing, instead of Debian unstable.
- [LTS is not] Cutting Edge: Instead of doing an automatic full package import from Debian unstable, we will do it from Debian testing. The benefit we gain from not introducing new bugs and/or regressions outweighs the new features and/or fixes we often get from unstable.
- We reserve the right to selectively pull in updates from unstable, if we believe the stability of the package in Debian is better than what is in the current Ubuntu archive.
The obvious corollary is that non-LTS releases are based, for the most part, on Debian unstable.
This appears to have changed recently per https://wiki.ubuntu.com/LTS: "Starting with the 14.04 LTS development cycle, automatic full package import is performed from Debian unstable"
https://web.archive.org/web/20131030153921/https://wiki.ubuntu.com/LTS
Ubuntu releases are based first and foremost on the previous Ubuntu release.
For a portion of the Ubuntu release cycle, source packages which are not locally modified in Ubuntu are auto-synced from Debian. Normally this auto-sync is from Unstable but for some earlier LTS releases it was from testing instead. It is also possible for packages to be manually synced from Debian either because local modifications in Ubuntu are no longer needed or because there is a desire to sync a newer version from Debian experimental (or unstable in the case of releases where the auto-sync was from testing).
Nowadays package updates (whether from the auto-sync, manual sync or from uploads) initially go into a special section of the archive called "proposed", they only get transferred to release when they pass certain checks.
Furthermore many of the core packages in Ubuntu are locally modified and Ubuntu is often ahead of Debian on versions for such packages.
Normally, Ubuntu is based on Unstable. LTS versions are based on Testing, which will eventually become stable. Precise Pangolin is based on Wheezy. Wheezy was Testing at the time Precise was released, but now Wheezy is stable.
(Note: if I am wrong, I hope someone corrects me, else I mislead people.)
