GNU is an acronym for "GNU's Not Unix". It is a free and open-source operating system that was developed as an alternative to proprietary Unix systems. The GNU project was launched by Richard Stallman in 1983 with the goal of creating a complete Unix-like operating system composed entirely of free software. 

The GNU project has developed a wide range of software tools and utilities, including the GNU Compiler Collection (GCC), which is a collection of compilers for various programming languages, and the GNU Core Utilities (coreutils), which provide essential command-line tools for Unix-like systems. These software components, along with the Linux kernel, form the basis of many popular operating systems, such as GNU/Linux distributions.

The GNU project is known for its strong emphasis on software freedom and the principles of copyleft. The GNU General Public License (GPL) is a widely used software license that ensures the freedom to use, modify, and distribute software, while also requiring that any modifications or derivative works be released under the same license.

Overall, GNU represents a significant contribution to the open-source software movement and has played a crucial role in the development of free and open-source software.