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 GPL is the most popular and well-known example of the type of strong copyleft license that requires derived works to be available under the same copyleft. Under this philosophy, the GPL grants the recipients of a computer program the rights of the free software definition and uses copyleft to ensure the freedoms are preserved, even when the work is changed or added to. This is in distinction to permissive free software licenses, of which the BSD licenses are the standard examples.

The GNU Lesser General Public License (LGPL) is a modified, more permissive, version of the GPL, originally intended for some software libraries. There is also a GNU Free Documentation License, which was originally intended for use with documentation for GNU software, but has also been adopted for other uses, such as the Wikipedia project.

The Affero General Public License (GNU AGPL) is a similar license with a focus on networking server software. The GNU AGPL is similar to the GNU General Public License, except that it additionally covers the use of the software over a computer network, requiring that the complete source code be made available to any network user of the AGPLed work, for example a web application. The Free Software Foundation recommends that this license is considered for any software that will commonly be run over the network.

The text of the GPL is not itself under the GPL. The license's copyright disallows modification of the license. Copying and distributing the license is allowed since the GPL requires recipients get "a copy of this License along with the Program". According to the GPL FAQ, anyone can modify the license as long as they use a different name for the license, not mention "GNU" and remove the preamble. The preamble can be used in a modified license with permission of the FSF. However, the only license to have permission of the preamble was version 1 of the Affero General Public License.

BSD license

BSD licenses represent a family of permissive free software licenses. The original was used for the Berkeley Software Distribution (BSD), a Unix-like operating system after which the license is named. The original owners of BSD were the Regents of the University of California because BSD was first written at the University of California, Berkeley. The first version of the license was revised, and the resulting licenses are more properly called modified BSD licenses. Permissive licenses, sometimes with important differences pertaining to license compatibility, are referred to as "BSD-style licenses". Several BSD-like licenses, including the New BSD license, have been vetted by the Open Source Initiative as meeting their definition of open source.

The licenses have few restrictions compared to other free software licenses such as the GNU General Public License or even the default restrictions provided by copyright, putting it relatively closer to the public domain.

The BSD License allows proprietary use, and for the software released under the license to be incorporated into proprietary products. Works based on the material may be released under a proprietary license or as closed source software. This is the reason for widespread use of the BSD code in proprietary products, ranging from Juniper Networks routers to Mac OS X.

It is possible for something to be distributed with the BSD License and some other license to apply as well. This was in fact the case with very early versions of BSD itself, which included proprietary material from AT&T.

Apache License

The Apache License (Apache Software License previous to version 2.0) is a free-software license authored by the Apache Software Foundation (ASF). The Apache License (versions 1.0, 1.1, and 2.0) requires preservation of the copyright notice and disclaimer, but it is not a copyleft license — it allows use of the source code for the development of proprietary software as well as free and open source software.

All software produced by the ASF or any of its projects or subjects is licensed according to the terms of the Apache License. Some non-ASF software is licensed using the Apache License as well: as of July 2009, over 5000 non-ASF projects located at SourceForge.net are available under the terms of the Apache License.

Licensing conditions

Like any free-software license, the Apache License allows the user of the software the freedom to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software.

The Apache License does not require modified versions of the software to be distributed using the same license nor even that it be distributed as free/open-source software. The Apache license only requires that a notice is kept informing recipients that Apache licensed code has been used. Thus, in contrast to copyleft licenses, recipients of modified versions of Apache licensed code do not necessarily also get the above freedoms. Or considering the situation from the Apache licensees perspective, they receive the freedom to use the code in any way they want, including using it in closed source products (cf Paragraph 4).

Two files that must be put at the top directory of redistributed software packages:

• LICENSE - a copy of the license itself.
• NOTICE - A "notice" text document listing the names of licensed libraries used, together with their developers.

In every licensed file, any original copyright or patent notices in redistributed code must be preserved, and in every licensed file changed a notification must be added stating that changes have been made to that file.

GPL compatibility

The Apache Software Foundation and the Free Software Foundation (FSF) agree that the Apache License 2.0 is a free software licence, compatible with version 3 of the GNU General Public License (GPL). ^[5]

However, the Free Software Foundation considers all versions of the Apache License (as of 2007) to be incompatible with the previous GPL versions 1 and 2.