Every modern computer runs a suite of programs, the operating system, that translates between the user's actions (such as typing on a keyboard or moving and clicking a mouse) and the computer's response (entering data in a file, moving a cursor on the screen). Most operating systems--from PC operating systems such as Windows and MacOS, to mainframe operating systems such as Solaris and other versions of Unix--are proprietary; they are licensed in such a way that users are actively forbidden from sharing them, or from discovering how they work and improving them. An analogy with cooking is useful. Proprietary software is analogous to packaged food with no ingredients listed, and with restrictions on the way the food may be served: You are forbidden from adding additional ingredients, and often even from combining the food with food made by other manufacturers. Proprietary software makes financial sense in some settings, but is at odds with academic freedom, and can even be used to restrict the personal freedom of individuals.3
In 1984, Richard Stallman set out to create a Free operating system to replace Unix. Stallman settled on the recursive acronym ``GNU'', short for ``GNU's not Unix'', as the name of his project. His vision of ``Freedom'' was similar to the recipe model of cooking: Neighbors may trade recipes freely, examine the ingredients and adjust them to their own tastes, and redistribute their modified recipes however they like. In this way, everyone's innovation benefits the whole community of cooks (or computer users). In spirit, this model is much closer to the academic ethic, and to American populism: Computers and their programs should be tools to improve everyone's access to information and quality of life, not merely the conduits through which entertainment and information are restrictively distributed for the profit of a wealthy few.
By the late 1980s, the GNU project had created a nearly complete operating system, consisting of utilities for running a computer system and creating new software. The one major piece lacking was the kernel, the single program that directly controls the computer's hardware and allocates resources to the various programs running at a given time. In a now-famous Usenet posting from 1990, Linus Torvalds announced his intent to create a Free Unix-like kernel that would run on the Intel 386. By the early 1990s (the days of DOS and Windows 3.1), there was a growing community of PC users who were running an operating system consisting of GNU with a Linux kernel. Eventually (after heated debate in the Free software/Open Source community) this operating system became known as ``Linux'', a name that omits the GNU project's foundational role.
There is a basic philosophical difference between ``Free'' and ``Open Source'' software. In brief, the Open Source community is concerned primarily with peer review and technical quality of software, while the Free software community in addition regards the ethical aspect of software sharing as central. Both communities are concerned with individuals' rights to use their computers productively for any purpose, and with their right to distribute work to the community in any way that does not violate other users' rights to do the same.
At this writing, GNU/Linux is available in several forms at low or no cost. A ``Linux distribution'' (which is what most people mean by ``a version of Linux'') consists of a few thousand programs and data files, organized in a more-or-less standard fashion. The web URLs at the end of this note include sites containing detailed documentation on all aspects of GNU/Linux.