GNU is the name of a computer operating system. The name is short for GNU's Not Unix. Richard Stallman leads the project working on GNU, known as the GNU Project. It was created to replace the UNIX operating system with all free and open source software. The popular Linux operating systems made using Linux kernel consist large number of GNU tools. Due to this reason, many projects and developers call the Linux based operating systems GNU/Linux.
The GNU project was started by Richard Stallman in 1983. His aim was to create a computer system fully free to modify, share and publish modified work. He and a group of developers started with creating copies, of each piece of Unix software, which are free to modify and share. The remaining part of GNU was the kernel, called the GNU Hurd, which is not yet completely built. The more popular Linux kernel is used instead.