GNU General Public Licence
The GNU General Public Licence (GPL) is issued by the Free Software Foundation (FSF).
The GPL grants users the following freedoms:
- The work may be used without any restriction for any purpose. Commercial use is expressly intended.
- Copies of the work may be distributed free of charge or for a fee provided this is noted in the licence and on condition that recipients are granted the same rights.
- The work may be modified provided such modifications are identified as such. Modified works may be distributed only under a licence identical to that which governs the original work.
Copyleft principle
Any derivatives of a work licensed under a GPL may be distributed by licensees only if the derivative works are licensed under GPL terms and conditions identical to those which govern the original work. This applies only to licensees not to copyright holders (i.e., authors or parties to whom the rights have been assigned). The latter are entitled also to distribute the work under any other non-exclusive licence. This principle can be found also in the other GNU licences (LGPL and GFDL), and in some of the Creative Commons licences where it is known as share alike.
Source: Wikipedia
