In 1992 Linus Torvalds, then a student at Helsinki University, began writing the "Linux" kernel - a 32-bit kernel for UNIX-like operating systems. The kernel development advanced rapidly and was released under the GPL licence starting from an early stage. To complete the system and make it into a usable UNIX system, the Linux developers used various existing user-land utilities and libraries from the GNU project and other sources (such as the X-Windows system), and wrote a few user-land utilities from scratch.

From an early stage, this entire system was dubbed "Linux" as well. Richard Stallman instead has advocated the name "GNU/Linux" (pronounced "ggnoo-Linux") which acknowledges the fact that the GNU project contributed the lion's share of the system. (as well as some pre-requisites of the Linux kernel itself). Most people haven't consistently followed this piece of advice.

The importance of the Linux kernel was that it was the last brick in materialising a fully GNU system. Since GNU tools tend to be more complete, feature-rich and generally superior to tools of other systems, this has made Linux one of the most powerful UNIX systems available. Nowadays, most UNIX servers out there and almost all workstations run the GNU/Linux system. Linux was, thus, the spearhead that guided the acceptance of free software into the mainstream.

Debian GNU/Linux was a Linux distribution that was eventually endorsed by the GNU project. One of the things that made it unique was the fact it distinguished between "free" and "non-free" packages as far as the user is concerned. The guidelines for determining which software is "free" in the Debian sense were phrased by Bruce Perens.

Note that they deviate from the free software definition (which was phrased later on) and include some licences that are not free. I.e: "Debian Free" is a superset of free software according to the Stallman definition.

This fact is important because later on, the Debian Free Software Guidelines formed the basis for the open-source definition.