COPYLEFT LICENCES

Most software is copyrighted, that is, "all right reserved". Thanks to a pun, the word copyleft was coined. It uses "left" as opposed to "right", as in political terminology, but also "left" as in the verb "to leave" with the sense of being "not constricted", "let free".
Yet any type of freedom, if it is to be granted, needs the observance of some rules. What if I produce free software and somebody uses it (I allow him to do that) and then they apply for a copyright on it?
This is the reason why "licenses" have been provided in order to protect and grant the rights of free use. Practically, to be legally protected, the creators of free software state: "©This software is released under licence X".

The main copyleft software licenses have been developed by the Free Software Foundation:

GNU GPL

The most widely used licence for open/free software is the GNU General Public Licence, GNU GPL, formulated by Richard Stallman and Eben Moglen in 1989. It aimed at distributing the programs created within GNU project. The latest version, 2, dates back to June 1991. (View http://www.gnu.org/licenses/gpl.html, 10-12-2005).
The kernel of GPL is as follows:

	the licence is applied to any program which, under the entry copyright, indicates GPL licence;
	the use of program is permitted;
	the alteration, copy and redistribution of software is allowed both in its original form or modified, both free of charge or not, provided that each copy shows the same licence and that the source code is available in case alterations wish to be made by the user.

GNU LGPL

The GNU Lesser General Public License - GNU LGPL - allows the creation of links between free and proprietary software. It has been mainly formulated in order to allow the use of repositories containing free and non-free programs.