4.1 GPL

GPL (GNU Public License) is, by far, the most well known open source license. The original license can be found at http://www.gnu.org/copyleft/gpl.html.

Let's interpret the points.

• Point 1. This means that you are free to copy and distribute a GPLed program package. It is not okay to remove certain files (such as the license itself) in the package before redistribution. It is okay to make money from physically distributing the copies, as well as to offer warranty work. This is rather important, as it means that one can still make money from so-called free software.
• Point 2. You can change the source code and make derived work. However, to copy and distributed the modified work, you need to follow some rules. You have to indicate that you have changed the source code. Your derived work also automatically GPLed. And, interactive programs must display a notice that summarizes the GPL.
• Point 3. You can copy and distribute a GPLed executable given that you let the recipient acquire, or at least know how to acquire, the source code. The distribution of the source code must be free of charge for profit.
• Point 4. A recipient of a GPLed program cannot change the license.
• Point 5. If you don't accept the GPL terms, you can still use the program, but not to modify or distribute the program. Any modification or distribution automatically means that you have accepted all the terms of the GPL!
• Point 6. If you redistribute a GPLed program, the license must remain as the original. You cannot impose further restrictions.
• Point 7. You cannot use the GPL as a reason to contradict work that is already under another license. In other words, the GPL does not give you protection to contradict other licenses.
• Point 8. Any known regional patent license contradiction must be disclosed so that it is the responsibility of the distributor to observe such restrictions.
• Point 9. The GPL is versioned, and an author can choose which version to use.
• Point 10. The incorporation of GPLed work into work that are non-GPLed requires explicit and written permission.
• Point 11 and 12. Legalese to protect the author and distributor from getting sued if bad things happen.