GNU is a Unix-like operating system that comes with source code that can be copied, modified, and redistributed. The GNU project was started in 1983 by Richard Stallman and others, who formed the Free Software Foundation. Stallman believes that users should be free to do whatever they want with software they acquire, including making copies for friends and modifying the source code and repackaging it with a distribution charge. The FSF uses a stipulation that it calls copyleft. Copyleft stipulates that anyone redistributing free software must also pass along the freedom to further copy and change the program, thereby ensuring that no one can claim ownership of future versions and place restrictions on users.