1. GNU General Public License (GPL)
OBJECTIVES
Guaranteeing freedom of the users utilizing the GPL licensed software, where by freedom is not only meant the free use of it but also the chance of modifying and sharing the software.
Protecting the authors of the Software from possible damage provoked by other programmers modifications on the same software.
Protecting the software from the threat of the patents; Since someone could modify the source code of a software and patent it.
HISTORICAL EVOLUTION
EMACS General Public License
1988 GNU GPL v1
GNU GPL v2 and LGPL
2007 GNU GPL v3

2. GNU GPL PARADIGM PER EXELLENCE OF THE FREE SOFTWARE LICENSE
COPYLEFT: "Anyone can copy and distribute the letteral copies of this document but modification is not permitted"
IT IS A DOCUMENT BASED ON COPYRIGHT
IT GRANTS THE USERS PERMISSION TO MODIFY THE PROGRAM, COPY IT AND REDISTRIBUTE IT WITH OR WITHOUT MODIFICATION, WITH OR WITHOUT CHARGE
THE USER IS REQUIRED TO MAKE AVAILABLE THE SOFTWARE SOURCE CODE TO PEOPLE WHO HAVE RECEIVED A COPY OR, ALTERNATIVELY, ACCOMPANY THE SOFTWARE WITH A WRITTEN OFFER TO MAKE SOURCE CODE AVAILABLE UPON REQUEST AT NOMINAL PRICE.

3. GNU GPL
It is PERSISTENT because it imposes a bond in the act of re-distributing it: If the user distributes the copies of the Software, it must be done according to the GPL terms. Basically, the text and the GPL software must be given along with the source code and the instructions to get it at a nominal price.
It is VIRAL because in the text it defines a particular interpretation of "derived code", such that generally speaking, unifying a programme covered by GPL, with another programme covered by a different license, basically means that it can only be distributed under the GPL license or else it cannot be distributed at all. In the first case the other license is compatible to the GPL and in the second case it is not compatibleSTRONG COPYLEFT

4. ABSENCE OF LIABILILITY IN CASE OF DAMAGE
GNU GPL NO WARRANTY
ABSENCE OF WARRANTY
The whole risk regarding the quality and performance of the programme is to be ascribed to the purchaser
ABSENCE OF LIABILILITY IN CASE OF DAMAGE
some of the exemplary sort of damage deriving from the use or the impossibility of using the programme" (data lost or incompatibility with others programmes).

5. GNU GPL v3 NOT COMPATIBLE WITH GNU GPL v2
it is impossible to combine the code realesed under the version 3 with the one of the v2
defense against circumvention of GPLv2 known as TIVOIZATION: It consists in distributing the source code along with a program that runs on a proprietary platform and precludes hardware to run modified versions of a given program, thus making void the freedom to modify.

6. GNU GPL v3
neutralization of some negative effects on software freedom by laws on Digital rights management (DRM)
defense against agreement forms so as to protect it from any patents infringement
the removal of any form of the patent from the free software or in the case a patent is owned by a proprietor, the default ceasing of all the rights belonging to the users of the programme.