Debian is a popular and influential GNU/Linux distribution. Questions here should generally be about Debian Server.
Debian GNU/Linux its one of the oldest and more popular Linux distributions. The Debian project was started in 1993 by Ian Murdock (the name Debian is a junction of Debra + Ian, his girlfriend at the time). Debian strongest points are the portability, stability and commitment to the free software ideal.
The current Debian version (6.0, code named Squeeze) has more than 29.000 software packages available on nine platforms (including SPARC, ARM, PowerPC), and 2 platforms supporting the FreeBSD kernel (Debian GNU/kfreebsd on i386 and amd64). The Debian APT package management provides dependency management and strong update and upgrade capabilities.
Debian maintains normally 3 versions of its repositories:
stable (the current release),
testing (the next release),
unstable (new packages and versions). After debian makes a new release, the previous
stable release is maintained for one year, and is re-labed
oldstable. Squeeze also included the
backports repository where some software from the
testing will be made available to the
stable release, and
updates (previously named
volatile) to deal with software that evolves too fast. Debian also makes point releases with aggregated updates and updated tools, those keep the release name but update the version like Debian 6.1 for example.
Debian has a Social Contract with its users and will only distribute on its main distribution software that is compatible with the Debian Free Software Guidelines.Those two devices were created on the early days of the distribution and also are the basis for Open Source Initiative's Open Source Definition.
The Debian project organization counts with the Debian Project Leader, the Debian Project Secretary and the Debian Developers. The developers are responsible for the releases, packaging software, managing the FTPs, security and so on. Each Project Leader is elected for a period of one year and any Developer can be a candidate.
Ubuntu Linux uses Debian as its base, as well as other well known distributions like Maemo, Mint, Knoppix and several others. The Wikipedia article has also more details about the distribution, as well as Debian's official site.