Debian is an extraordinarily popular Linux distribution. It was developed originally by Ian Murdock and his wife, Debra Murdock. Debian is quite strict with adherence to the philosophies of Unix and free software as well as using collaborative software development and testing processes. It forms the basis of many other linux distributions such as Ubuntu, Linux Mint, Android, and others. It is focused on stability and security, and is therefore very popular on niche devices such as network storage drives, servers, mainframe computers and other critical systems. Unlike many other popular distributions, Debian is not supported by the private sector, and is completely built by the open source community.
Debian is composed of nearly 2 billion lines of code, supporting up to 11 different computer architectures. Debian is also one of the first distributions to use packages, the APT package, developed for the system. One of the most redeeming points of Debian is it's powerful, robust package handling, which has resulted in a glut of available software developed for it.
The default repository, called "stable" is very conservative, keeping old software versions for as long as possible and updating them only when the new distro release comes (about once per 1,5-2 years). Instead of updating the software to fix bugs, old versions are patched to ensure stability. This has led to a degree of criticism among some, but, thanks to the flexible package management system Debian provides, it is possible to use a more active repository for the whole system or even separate packages.
The repository is also divided in several categories according to the software's licensing status. The "free" category, the only one accessible by default, only includes software classified as free software which can be used by itself. The "contrib" category includes free software that may require proprietary resources to work, while the "non-free" category includes some proprietary software that may be required for work.