Debian is a free and open-source computer operating system that uses a Unix-like kernel, typically Linux, alongside other program components. It was first introduced on August 16, 1993, by Ian Murdock, and is one of the oldest operating systems based on the Linux kernel. Debian is composed of free and open-source software, developed by the community-supported Debian Project, which is coordinated 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.

Debian is known for its stability and security, and it is a complete free operating system. It is offered in three main channels: testing (developing), stable, and unstable, with the stable channel being the most popular edition for personal computers and servers. Debian offers over 59,000 packages that support a wide range of capabilities, including editing documents, developing software, administering systems, connecting to networks, debugging packages, mixing sound, routing email, and playing games. All packages included with a Debian distribution are free, as outlined in the Debian Free Software Guidelines.

Debian is a volunteer project that has developed and maintained a GNU/Linux operating system for well over a decade. It is the rock on which Ubuntu is built, and Ubuntu builds on the Debian architecture and infrastructure and collaborates widely with Debian developers. However, there are important differences between the two, including a distinctive user interface, a separate developer community, and a different release process.