GPL
From FOSS Users
The GNU General Public License (GPL) is the most important copyleft-license for FOSS. By now it is available in version 3. The version 2 is still in active use while version 1 has no practical use any more.
The GPL (all versions) requires that the recipients of the software have the same freedoms as the remodifiers or redistributors. In particular the GPL protects the freedom to be allowed to make modified versions of the software. Since the machine code executed by the processor is virtually unreadable for humans, the GPL requires that the human-readable source code of the software is made available.
