- "The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program--to make sure it remains free software for all its users."
- - GPL V3
The GNU Public License (GPL) is the original and most influential FOSS license. It set the standard, requiring GPL software modifications to be made available under the same license, a more than fair exchange in return for all the value the user of GPL code receives for free.
Paradoxically to those that don't understand the value proposition the GPL offers and look for other somehow qualified but still open licenses, among the most successful FOSS from the business perspective of creating large ecosystems with large revenue streams is released under the GPL, including Linux itself. Indeed, to the surprise of some, there is not prohibition under the GPL against selling the binary of GPL software and/or providing support for any amount the market will bear, as long as you make the source available for free so the customers have the option of doing it themselves if they wish.
Version 3 of the GPL added protections against use with software locking.
The Lessor GPL (LGPL) is more flexible than the GPL, enabling one to "combine or link" with LGPL code without having the linked code also become subject to the GPL. It is mainly meant for operating system and library type linking, but is also used sometimes by those that wish to provide even more freedom to incorporate their software in large systems. The level of restriction if so small that it approaches the near complete freedom of the BSD License. The relevant text added to the GPL is shown below.
"As an exception to the Sections above, you may also combine or link a "work that uses the Library" with the Library to produce a work containing portions of the Library, and distribute that work under terms of your choice, provided that the terms permit modification of the work for the customer's own use and reverse engineering for debugging such modifications."
The master copies are found here www.gnu.org/copyleft/gpl.html.