The GNU operating system is a complete free software system, upward-compatible with Unix. GNU stands for “GNU's Not Unix”. It is pronounced as one syllable with a hard g. Richard Stallman made the Initial Announcement of the GNU Project in September 1983. A longer version called the GNU Manifesto was published in March 1985. It has been translated into several other languages.
Here is a link to the GNU project manifesto, written by Richard M. Stallman.