Debian vs Ubuntu
Debian and Ubuntu are free Linux distributions using the apt package management system. Ubuntu builds on the foundations of Debian architecture and infrastructure, with a different community and release process .
Ubuntu is specifically designed to be easy for inexperienced users to use. Initial configuration of Debian may be more difficult. Ubuntu's early motto was "Linux for human beings", while Debian describes itself as "the universal operating system." The decision to use one or the other may also hinge on the relative importance of new, possibly unstable software versus old reliable software.
Community is probably the biggest distinguishing feature besides distribution "flavor". The Ubuntu forums are more accessible to newcomers, while Debian forums are more technical. Both distributions depend heavily on a large community of volunteer open-source software developers and users who provide free support for each other while using the software. Ubuntu is based on Debian so it relies on Debian community as well as on Ubuntu community. Ubuntu is also sponsored by (and was created by) the company Canonical, which offers fee-based support services for Ubuntu, whereas Debian is developed entirely by the community.
 Target Architecture
Ubuntu is aimed at desktop and server users using the Intel x86, x86-64, and (some versions) powerpc architectures only, whereas Debian runs not only on these but also on a huge range of hardware from embedded or handheld devices using ARM to MIPS processors to platforms such as Sparc, Intel's IA64 and Alpha, to name a few. There are advantages and disadvantages to supporting such a wide range of different hardware; one disadvantage Debian faces is that it takes more time and resources to ensure a piece of software works on all its architectures.
 Standard Releases
Both Ubuntu and Debian have standard releases that allow users and contributers to use the system. Most of the release separation is managed by the apt source.
 Debian Unstable (a.k.a. Sid)
The Debian Unstable branch is an opportunity for developers and experienced computer users to use, test, and develop the very latest open source software. It is not recommended for beginners or for anyone for whom reliability is a priority. While the quality of software in Debian Unstable is usually very high, the branch is highly unstable in the sense that things change rapidly and constantly; developers can't rely on libraries that are available (and working) one day to be available the next, and users can't rely on their correctly-configured and smooth-running system not to need additional configuration and workarounds every time something changes.
 Ubuntu Releases (non-LTS)
These releases are based on the Debian Unstable branch, and are made every six months (although every fourth release becomes an LTS release, see below).
Ubuntu releases (whether LTS or non-LTS) include software that is more up-to-date than the software in the Debian Unstable branch when developers believe it will benefit their users. For example, the version of Ubuntu released in April 2010 contained Firefox 3.6, while Debian Unstable still included Firefox 3.5 (with Iceweasel branding).
Ubuntu's goals are to start with the most up-to-date software that Debian has to offer, release new stable versions regularly on a predictable 6-month cycle, and make an operating system that is easy to use and set up for beginners. To this end, Ubuntu adds a number of features of its own aimed at making configuration easier for the uninitiated, offering graphical configuration tools in place of command-line or configuration files in places.
Ubuntu puts a lot of its own effort into making the Unstable branch of Debian as reliable as it can given the newer software that it uses and its more rapid release cycle. After being pulled from Debian Unstable, it undergoes some Ubuntu-specific changes and a series of testing and bug-fixing before it is declared as a release. Some of the work that Ubuntu does in testing newer software results in patches that make it back to the software maintainers, which in turn helps improve Debian (and other distributions) for its future releases.
The latest non-LTS release version of Ubuntu is 13.04. In April 2012 Ubuntu 12.04 LTS was released.
 Debian Testing
The Debian Testing branch represents the state of the upcoming Debian release, before it is Stable. Most of the time, packages from Debian Unstable (a.k.a. Sid) will transfer across to Testing, usually after 10 days, as soon as they meet a stringent set of guidelines, such as having no new known "release-critical" bugs and having all dependencies installable and satisfied. Thus, some of the more immediately obvious bugs and problems in Debian Unstable will be prevented from entering Testing. Nevertheless, Debian Testing is still inherently unstable due to constantly changing most of the time, and it can be affected by interoperability problems between packages, especially when a major change to a library has recently entered.
Prior to a Debian Stable release, Debian Testing will become 'frozen', accepting only fixes to existing serious bugs. This occurs for a few months, with the aim of resolving all "release-critical" bugs. After this time, a new Debian Stable branch is created from Debian Testing, and Testing is unfrozen again and given a new codename, in preparation for testing the next release.
"Wheezy" is the codename for current Debian Testing.
 Ubuntu LTS (Long Term Support) Releases
Each Ubuntu LTS (Long Term Support) release will initially be based on the Debian Testing branch. These releases take Ubuntu's approach of making Linux accessible to inexperienced users, and constitute a release that is going to be kept stable for five years (before 12.04 three years) into the future, receiving security updates and some occasional bug fixes during that time, but remaining otherwise unchanged.
Essentially, an Ubuntu LTS release is made in the same way as a non-LTS release, except that it will be kept stable and supported for a lot longer, including by Canonical's own paid support services. In addition, it was decided that Ubuntu LTS releases should initially be based on Debian Testing rather than the similarly unstable but sometimes buggier Debian Unstable.
It is intended that every fourth Ubuntu release shall become an LTS release, thus Ubuntu LTS releases are made every two years. The most current Ubuntu LTS release is Ubuntu 12.04.
 Debian Stable
These releases are very reliable. A "Debian Testing" is branched to form a new "Debian Stable" release after it has been frozen for some months, tested extensively and very few problems remain.
Debian Stable is supported by security updates, and fixes to extremely major bugs, until the next Debian Stable release aimed at 2 years later, and then for an additional year (similar to Ubuntu LTS). During this time it does not, however, receive any new versions of software, nor any bug-fixes except for a small number of bugs deemed major enough and for which a fix is unlikely to change any relied-upon behaviour.
Because of this, users wishing to test or use the latest versions of software with Debian Stable will need to either compile it themselves, or use a 'backports' repository providing some newer versions of software which has been compiled to be compatible with Debian Stable. Developers, or technically-adept people who know how to fix problems and want the latest software, may like to use 'Testing' or 'Unstable' instead.
"Wheezy" is the current Debian Stable release.
 Repositories for updating specific packages
 Ubuntu PPAs
Ubuntu supports Personal Package Archives, which are mini repositories that can be installed to get a specific package or set of packages that aren't in the main repositories, or to update a package or set of packages to a newer version than the version in the repositories. Ubuntu users may choose an LTS release, but then install a PPA for a specific program they would like to keep more up to date, e.g. Firefox 5 on Ubuntu 10.04 LTS.
PPA repositories co-exist with the Official Ubuntu Package Repository. Debian can still use PPA Repositories but they are considered to be 3rd party repositories.
 Debian Backports
Debian Backports are similar to Ubuntu PPAs, insofar as getting an updated package on a stable / LTS operating system goes.
"You are running Debian stable, because you prefer the Debian stable tree. It runs great, there is just one problem: the software is a little bit outdated compared to other distributions. This is where backports come in."
"Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates) in a stable environment so that they will run without new libraries (whenever it is possible) on a Debian stable distribution."
Backports can be used in a similar way to PPAs, e.g. to install IceWeasel 5 on Debian 6.0.
 Other Releases
The Debian community continues to support a release of Debian Stable even for one year after a newer Stable release is made. When this happens, the old Stable release is dubbed "Oldstable". The current "Oldstable" release is called Lenny.
There are other variations of Debian and Ubuntu, suitable for users with different computer processor types, older computers, or other specific needs.