Debian ( ) is a computer operating system composed of free and open source software. The primary form, Debian GNU/Linux, is a popular and influential Linux distribution. It is distributed with access to repositories containing thousands of software packages ready for installation and use. These packages are distributed under free software / open source licenses. Debian is known for strict adherence to the Unix and free software philosophies as well as using collaborative software development and testing processes.

The Debian Project is governed by the Debian Constitution and the Social Contract which set out the governance structure of the project as well as explicitly stating that the goal of the project is the development of a free operating system.

Thus, the Debian Project is an independent decentralized organization; it is not backed by a company like other Linux distributions such as Ubuntu, openSUSE, Fedora, and Mandriva. The cost of developing Debian 4.0 etch, assuming paid programmers from a single organization and based on source lines of code, has been estimated to be close to US$13,000,000,000. As of April 2, 2009, Ohloh estimates that the Debian GNU/Linux project, assuming a $55,000 average salary, would cost $819,274,547 to redevelop from scratch.

Many distributions are based on Debian, including Ubuntu, MEPIS, Dreamlinux, Damn Small Linux, Xandros, Knoppix, BackTrack, Linspire, sidux, Kanotix, Parsix and LinEx, among others.

Debian is known for an abundance of options. The current stable release includes over twenty five thousand software packages for twelve computer architectures. These architectures range from the Intel/AMD 32-bit/64-bit architectures commonly found in personal computers to the ARM architecture commonly found in embedded systems and the IBM eServer zSeries mainframes. Prominent features of Debian are the APT package management system, repositories with large numbers of packages, strict policies regarding packages, and the high quality of releases. These practices allow easy upgrades between releases as well as automated installation and removal of packages.

The Debian standard install makes use of the GNOME desktop environment. It includes popular programs such as OpenOffice.org, Iceweasel (a rebranding of Firefox), Evolution mail, CD/DVD writing programs, music and video players, image viewers and editors, and PDF viewers. There are pre-built CD images for KDE, Xfce and LXDE as well. The remaining discs, which span five DVDs or over thirty CDs, contain all packages currently available and are not necessary for a standard install. Another install method is via a net install CD which is much smaller than a normal install CD/DVD. It contains only the bare essentials needed to start the installer and downloads the packages selected during installation via APT. These CD/DVD images can be freely obtained by web download, BitTorrent, jigdo or buying them from online retailers.