GNU GNU, which stands for GNU's not Unix, is a system for producing and distributing free software that's upwardly compatible with Unix. By free, the keepers of the nonprofit GNU Project mean a number of things: You can distribute software that falls under one of the GNU Public Licenses to whomever you like; you can have complete access to the program's source code; you can tweak and alter the software in any way you like; and you can redistribute your changes to the software community (either by charging for your distribution or by giving it away). The core, or kernel, of the popular Linux OS is the best-known program that falls under the GNU General Public License.