Debian is a UNIX-like operating system composed entirely of free software. Most of the software it contains is licensed under the GNU General Public License, and is packaged, developed, and maintained by teams of participants in the Debian program. It is famous for its guidelines, adherence to the spirit of Unix and free software, and the many choices it gives users. Many companies choose Debian because of security and stability considerations. Debian distributes security patches quickly; in terms of stability, in addition to providing support for up to 5 years, it also provides a smooth upgrade plan to the next version.