GNU (General Public License)

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The GNU General Public License (GNU GPL or GPL) is the most widely used software license, which guarantees endusers the freedoms to use, study, share (copy), and modify the software. 

                  Software that ensures these freedom-rights, is called free software. The license was originally written by Richard Stallman of the Free Software Foundation (FSF) for the GNU project.The GPL license can be used by anyone, to ensure the enduser's freedom rights (use, share, study, modify).

                It is the first copyleft license for general use. Copyleft means that derived works can only be distributed under the same license terms. Under this philosophy, the GPL grants the recipients of a computer program the freedom-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 . (source wikipedia)