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The GNU General Public License (GNU, GPL, or GPL) is a free software license originally written by Richard Stallman of the Free Software Foundation, which guarantees that users are free to use, share, and modify the software without paying anyone for it.

There are two important terms of the GPL that are required for proper usage. First, any developer who wants to copy and distribute a verbatim copy of a piece of GPL-licensed software must display a copyright notice, warranty disclaimer, intact GPL notices, and a copy of the GPL with the copy of the software they are distributing. This will ensure that they do not try to misrepresent software in any way. Second, if they make any modifications, expansions or innovations to the software before they distribute it, they have to include an explanation of all of the changes they made, in addition to all of the other notices required for a verbatim copy.

The GPL is a copyleft license, in which the license is protective or reciprocal for all works that are derived from the original. This copyleft concept distinguishes it from existing copyright law, in which an author is empowered to prohibit users from reproducing, adapting, or distributing their work without paying for the right to do so. Copyleft licenses like the GPL have been the most popular licenses in the free and open-source software arena. The Linux kernel is a popular example of software that achieved widespread acceptance and success due to the freedom and copyleft protections provided by the GPL.