GNU General Public License

The GNU General Public License (GNU GPL or simply GPL) is a widely used free software license, originally written by Richard Stallman for the GNU project. The latest version of the license, version 2, was released in 1991. The GNU Lesser General Public License (LGPL) is a modified version of the GPL, intended for some software libraries

The GPL grants the recipients of a computer program the following rights:

• the right to run the program, for any desired purpose.
• the right to study how the program works, and modify it. (Access to the source code is a precondition for this)
• the right to redistribute copies.
• the right to improve the program, and release the improvements to the public. (Access to the source code is a precondition for this)

In contrast, the end-user licenses (EULA) that come with proprietary software generally only grants the end-user the right to copy the software onto a limited number of computers. The terms and conditions of such license agreements may even attempt to restrict activities normally permitted by copyright laws, such as reverse engineering.

The primary difference between the GPL and more "permissive" free software licenses such as the BSD License is that the GPL seeks to ensure that the above points are preserved in copies and in derivative works. It does this using a legal mechanism known as copyleft, invented by Stallman, which requires derivative works of GPL-licensed programs to also be licensed under the GPL. In contrast, BSD-style licenses allow for derivative works to be redistributed as proprietary software.

By some measures, the GPL is the single most popular license for free and open source software. As of April 2004, the GPL accounted for nearly 75% of the 23,479 free-software projects listed on Freshmeat, and about 68% of the projects listed on SourceForge. (These sites are owned by OSTG, a company that advocates Linux and the GPL.) Similarly, a 2001 survey of Red Hat Linux 7.1 found that 50% of the source code was licensed under the GPL, and 1997 survey of MetaLab, then the largest free-software archive, showed that the GPL accounted for about half of the licenses used. Prominent free software programs licensed under the GPL include the Linux kernel and the GNU Compiler Collection (GCC). Some other free software programs are dual-licensed under multiple licenses, often with one licenses being the GPL.