Ubuntu is a derivative of Debian. It uses the same package format.
In what ways is Ubuntu different from Debian?
Ubuntu – How is Ubuntu different from Debian
debian
Related Solutions
is possible but not recommended it can cause your system to become unstable. Do not use any foreign repositories. Look for backports or compile the package from source, using the deb-src repositories, if you need.
A "Debian release" itself isn't one thing, there usually different releases of Debian in the wild.
Debian has a branch called "unstable", or "sid", named after the kid from Toy Story who breaks his toys.
Debian also has a "stable" release, which is released when it's ready. These are usually released in longer intervals.
In between is a release called "testing", with versions of software newer than stable but older than unstable. Eventually "testing" is promoted to a stable release.
Typically during their development cycle, Ubuntu imports source packages from Debian Unstable (sid). At some point we stop automatic imports and instead pull in fixes manually and then release it as Ubuntu every 6 months. During LTS releases where stability is more important, Ubuntu will pull from the testing release instead.
After Ubuntu releases, there's a big sync up back with Debian and the process repeats.
Both Debian Stable and Ubuntu take Debian Unstable -> Testing -> and then make stable releases based off of that. The major difference between the processes is that Ubuntu is time-based, importing and making freezes based on the schedule and goals for that release, whereas Debian cycles at a slower rate and doesn't have a strict "We need to release regularly" goal. Ubuntu will also sometimes take things that are important to its users and package those directly, sometimes submitting those back to Debian if appropriate. I talk a bit about that here:
A long time ago Debian stable releases could take a really long time, sometimes up to 3 years: so long that by the time it was released some of the software was badly out of date. The past few releases have been much quicker, running about 1.5-2 years between - about the same interval as Ubuntu LTS releases, but since the schedule isn't the same, the Debian release may have newer software than the most recent Ubuntu LTS release (or vice versa).
Debian and Ubuntu packages are often binary compatible: programs built on one distribution can be copied to the other and will run successfully. But this relies on having the same libraries available that the package was built against, which is not always the case, either between an Ubuntu release and a Debian release or between two successive releases of the same distribution. Also, there's a lot more to distribution integration than just having the right libraries, so even if the program in the package has everything it needs to be run, the package could fail to install or run correctly because of any number of differences between the distributions: an Ubuntu package may not be usable on Debian because it requires upstart, for example.
In general, if you think the packaging should be the same between Debian and Ubuntu, it's still easiest to rebuild the binary packages for the exact toolchain of your release. You can easily import the source package and build it yourself or on in a PPA.
Every once in a while, the Debian and Ubuntu releases line up where we can share things like a GCC, libc6, or python version or whatever. That is of course great for the ecosystem because having things that closely aligned makes things easier for users, packagers, and upstreams. This is why the toolchain discussions at the Ubuntu Developer Summit usually take into account the state of Debian at the time.
Best Answer
So maybe its crazy for me to answer this, having just joined Canonical 3 months ago (today!) and having only run Ubuntu out of curiosity for the few years before joining Canonical. It might even be silly for me to answer it, given that I am on the server team, and Ubuntu definitely has a very large focus on making "Linux for Humans", ergo, the desktop.
To me, the release cycle is everything. Yes there are some things that will never go back to Debian, because these things are somewhat counter to Debian's philosophies. But these are by-products of the greater goal of usability.
When Ubuntu was started, the idea was simple. Debian was awesome then, and is still awesome today. I ran it exclusively for several years and it served me well on laptops, desktops, and especially on servers, being a server kind of guy. But that release cycle was so slow that all of the cool whiz bang stuff that people were producing on Linux was just not making it into the stable releases, and the unstable development release that had all of these things was un-installable (no official isos) and broken quite often.
So by saying "we're going to limit our focus to a couple of architectures, and a subset of packages" (the "main" archive in Ubuntu), the Ubuntu project was able to commit to releasing a tested, stabilized OS with all of that cool new stuff in it. They were also able to commit to carrying a bit of a delta from Debian that was highly focused on usability. By committing capital investment to it, Canonical was able to commit to having the technical staff available to make that happen.
One awesome part of that was that they (I say they, because I am not an Ubuntu member yet) could still keep a lot of the wide breadth of Debian software by creating the "universe" archive. Even better, a community (MOTU) grew up around that to make sure it received some stabilization before release as well.
So, to sum it all up with an analogy.. Ubuntu is to Debian, as your local restaurant is to the local farmer's market. Chef Ubuntu goes to the Debian farmer's market periodically, finds the best fresh ingredients, mixes them with his own special blend, and produces food for his intended audience. For people who enjoy cooking, they can, and do, just go down to the market and get what they need.