Ubuntu and Debian are closely related. Ubuntu builds on the foundations of Debian architecture and infrastructure, with a different community and release process.
Debian is "the rock upon which Ubuntu is built".
Debian is a volunteer project to develop a GNU/Linux distribution. Debian was started more than a decade ago and has since grown to comprise more than 1000 members with official developer status and many more volunteers and contributors. It has expanded to encompass nearly 17,000 "packages" of free and open source applications and documentation.
Debian's history, make-up, and position make it very good at certain things. Debian has a well-deserved reputation for integrated package management and access to a large list of free software applications. As a result, Debian has grown into one of the largest Linux distributions.
As a volunteer organization, Debian has historically been less good at making time-based or predictable releases, and has a difficult time providing accountability. Stable releases of Debian have been few and far between in recent years. The more raw unreleased versions of Debian do not provide security fixes for individual packages that are rapidly changing, but are incorporated in development work. While not important to many Debian users, these shortcomings have discouraged a number of potential Debian users. Still Debian is ported to more architectures than any other distribution, and allows for almost any combination of free software components to be mixed and matched as you wish.
The Ubuntu team hopes to address some of those issues and bring more people onto the Debian system.
Sponsored by Canonical, the Ubuntu project attempts to work with Debian to address the issues that keep many users from using Debian. Ubuntu provides a system based on Debian with frequent time-based releases, corporate accountability, and a more considered desktop interface. Ubuntu provides users with a way to deploy Debian with security fixes, release critical bug fixes, a consistent desktop interface, and to never be more than six months away from the latest version of anything in the open source world.
Ubuntu and Debian
Ubuntu and Debian are distinct but parallel and closely linked systems. The Ubuntu project seeks to complement the Debian project in the following areas:
Ubuntu does not provide security updates and professional support for every package available in the open source world, but selects a complete set of packages making up a solid and comprehensive desktop system and provides support for that set of packages.
For users that want access to every known package, Ubuntu provides a "universe" component (set of packages) where users of Ubuntu systems install the latest version of any package that is not in the supported set. Most of the packages in Ubuntu universe are also in Debian, although there are other sources for universe too. See the Ubuntu Components page for more detail on the structure of the Ubuntu web distribution.
Ubuntu makes a release every six months, and supports those releases for 18 months with daily security fixes and patches to critical bugs.
As Ubuntu prepares for release, we "freeze" a snapshot of debian's development archive ('sid'). We start from 'sid' in order to give ourselves the freedom to make our own decisions with regard to release management, independent of Debian's release-in-preparation. This is necessary because our release criteria are very different from Debian's.
As a simple example, a package might be excluded from Debian 'testing' due to a build failure on any of the 11 architectures supported by Debian 'sarge', but it is still suitable for Ubuntu if it builds and works on only three of them. A package will also be prevented from entering Debian 'testing' if it has release-critical bugs according to Debian criteria, but a bug which is release-critical for Debian may not be as important for Ubuntu.
As a community, we choose places to diverge from Debian in ways that minimize the difference between Debian and Ubuntu. For example, we usually choose to update to the very latest version of Gnome rather than the older version in Debian, and we might do the same for key other pieces of infrastructure such as X or GCC. Those decisions are listed as Feature Goals for that release, and we work as a community to make sure that they are in place before the release happens.
Many Ubuntu developers are also recognized members of the debian community. They continue to stay active in contributing to debian both in the course of their work on Ubuntu and directly in debian.
When Ubuntu developers fix bugs that are also present in debian packages -- and since the projects are linked, this happens often -- they send their bugfixes to the Debian developers responsible for that package in debian and record the patch URL in the debian bug system. The long term goal of that work is to ensure that patches made by the full-time Ubuntu team members are immediately also included in debian packages where the debian maintainer likes the work.
In Ubuntu, team members can make a change to any package, even if it is one maintained by someone else. Once you are an Ubuntu maintainer it's encouraged that you fix problems you encounter, although we also encourage polite discussions between people with an interest in a given package to improve cooperation and reduce friction between maintainers.
Freedom and Philosophy
Debian and Ubuntu are grounded on the same free software philosophy. Both groups are explicitly committed to building an operating system of free software.
Differences between the groups lie in their treatment of non-computer applications (like documentation, fonts and binary firmware) and non-free software. Debian distributes a small amount of non-free software from their Internet servers. Ubuntu will also distribute binary drivers in the "restricted" component on its Internet servers but will not distribute any other software applications that do not meet its own Ubuntu Licensing Guidelines.
Ubuntu and other Debian derivatives
There are many other distributions that also share the same basic infrastructure (package and archive format). Ubuntu is distinguished from them in a number of ways.
First, Ubuntu contributes patches directly to Debian as bugs are fixed during the Ubuntu release process, not just when the release is actually made. With other debian-style distributions, the source code and patches are made available in a "big bang" at release time, which makes them difficult to integrate into the upstream HEAD. Ubuntu patches are published automatically on an ongoing basis.
Second, Ubuntu includes a number of full-time contributors who are also debian developers. Many of the other distributions that use debian-style packaging do not include any active debian contributors.
Third, Ubuntu makes much more frequent and fresher releases. Our release policy of releasing every six months is (at the time of writing unique in the linux distribution world. Ubuntu aims to provide you with a regular stable and security-supported snapshot of the best of the open source world.