The GNU Lesser General Public License is a free software license published by the Free Software Foundation, or FSF. It is an alternative to both traditional copyright, which some consider to be too restrictive, and the original GNU General Public License, which others considered to be too permissive. The GNU Lesser General Public License (LGPL), first published in 1999, is different than the GNU General Public License, or GPL, in several ways. The most important difference being that programs licensed under the GNU Lesser General Public License can be accessed or used by any program, including copyrighted proprietary programs. Software licensed under the traditional GPL can only be used by or linked to by other free software.
OpenOffice.org is an example of a program created using the GNU Lesser Public License. The office suite is distributed for free as an alternative to Microsoft Office, and it uses its own file formats and extensions. However OpenOffice.org can also read and open most Microsoft Office files, and can save files to Microsoft Office compatible formats. This interaction with proprietary software is key to the GNU Lesser General Public License. An example of a program licensed under the traditional GNU GPL would be Audacity. This free audio editor cannot interact with any proprietary audio formats such as WMA (Windows Media Audio). Only other open-source audio formats, such as WAV, OGG and AIFF are compatible with the program.
The GNU Lesser General Public License is an example of copyleft. A copyleft is in contrast to a copyright. Copyrights exist to ensure no one can distribute, copy or adapt the program. Copyleft licenses allow anyone to reprogram, distribute or adapt software under the license without having to pay or ask for permission from the original creator. There are many free software licenses but not all are true copyleft licenses.
In order to be considered a copyleft license, the free software license must meet certain requirements. The most important of these is that it cannot be a permissive license. This means it cannot allow the reusing of the licensed software by proprietary, or copyrighted, software. Examples of permissive licenses include the MIT License or the PHP License. Neither the GNU GPL nor the GNU Lesser General Public License are permissive licenses. Any software created under those licenses cannot be modified to be included in software that has a more restrictive software license. For example, someone cannot take the code used to create OpenOffice.org, modify it and place it under a traditional copyright.