What are the most important open source licenses, and what type of license are they?

The GNU General Public License (GPL) is the most important and widespread open-source license.  Approximately 60% of all open-source software is under this license.  This includes well-known programs like Linux or Busybox. The GPL is the template for all licenses with a strict copyleft as well as version 2 (→ GNU General Public License, Version 2) and version 3 (→ The GNU General Public License, Version 3).


The GNU Lesser General Public License (LGPL) was earlier termed the "GNU Library General Public License" and was especially conceived for program libraries.  It has a restricted copyleft that enables the libraries to be linked with open-source software and proprietary software.


Version 2.0 of the Apache License now exists as a non-copyleft license.  Its most important program is the Apache Webserver.


The BSD license is the template for numerous non-copyleft licenses such as the MIT License, PHP License or OpenLDAP Public License that generally differ in name only. The operating systems NetBSD, Open BSD and FreeBSD can be used under this license.


The Common Public License (CPL) is a strict open-source license that was frequently used by IBM and is therefore widespread.  Microsoft licensed Windows Installer XML under the CPL. The Java development environment Eclipse can be used under the similarly named Eclipse Public License.


The Mozilla Public License (MPL) was developed to license the Netscape Communicator as open source software.  Because third-party components could not be released, the license only has a restricted copyleft according to which only changes in the original files must be subject to the MPL.  Since Mozilla programs are now also offered under different open-source licenses, the importance of the MPL has greatly decreased.


The European Public License (EUPL) is a license developed by the European Commission with a strict copyleft that exists in the 22 languages of the member states.  It is anticipated that this license will be increasingly used by public administrations for licensing independent developments.  The EUPL is compatible with the GPL (→ What is license compatibility?).


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