Wikipedia
Debian is a Unix-like computer operating system that is composed entirely of free software, most of which is under the GNU General Public License, and packaged by a group of individuals called the Debian Project. Three main branches are offered: Stable, Testing, and Unstable.
The Debian Stable Linux distribution is one of the most popular for personal computers and network servers, and has been used as a base for several other distributions. The Debian Testing and Unstable branches are rolling release and eventually become the Stable distribution after development and testing. Packages are first uploaded to Unstable, from which they migrate to Testing. When Testing is mature enough it becomes Stable.
The Debian Project was first announced in 1993 by Ian Murdock, Debian 0.01 was released on September 15, 1993, and the first stable release was made in 1996. The development is carried out over the Internet by a team of volunteers guided by the Debian Project Leader and three foundational documents: the Debian Social Contract, the Debian Constitution, and the Debian Free Software Guidelines. New distributions are updated continually, and the next candidate is released after a time-based freeze.
As one of the earliest Linux distributions, it was envisioned that Debian was to be developed openly in the spirit of the GNU Project. This vision drew the attention and support of the Free Software Foundation, which sponsored the project for one year from November 1994 to November 1995. Upon the ending of the sponsorship, the Debian Project formed the non-profit organisation Software in the Public Interest.