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The GPL, or GNU General Public License, is an open source license used by by the Linux kernel, all GNU software, Firefox, and many independent software projects.

Source code licensed under the GPL usually contains the following text at the top of each file:

 This program is free software; you can redistribute it and/or
 modify it under the terms of the GNU General Public License
 as published by the Free Software Foundation; either version 2
 of the License, or (at your option) any later version.
 
 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.
 
 You should have received a copy of the GNU General Public License
 along with this program; if not, write to the Free Software
 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.

[edit] Complete Text

See http://www.gnu.org/copyleft/gpl.html

[edit] Analysis

In short, the GPL guarantees the Free Software Foundation's four essential freedoms for software:

1. Freedom to run the program, for any purpose.
2. Freedom to study how the program works, and adapt it to your needs.
3. Freedom to redistribute copies so you can help your neighbor.
4. Freedom to improve the program, and release your own improvements to the public, so that the whole community benefits.

Access to the source code is a precondition for the second and fourth of these freedoms.

For GNU's explanation of this, see http://www.gnu.org/philosophy/free-sw.html.

[edit] See Also

• LGPL -- a more lenient variant intended for libraries to allow static linking
• BSD License and MIT License -- same idea, opposite coasts. Do whatever you want, but credit the authors; no requirement to share the source code, in contrast with the GPL.
• Free Software Foundation