1. COPYLEFT
THE TERM
The Term copyleft was forged upon the traditional copyright term by opposing the word right (which in English means both right meant as the collections of laws and the direction right as oppesed to left) to left (meaning the opposed direction of right and gone)
"Copyleft all right reversed" instead of "Copyright all rights reserved"
Left is also the past participle of to leave (to abandon, to allow)

2. WHAT IS COPYLEFT
General method for developing free software programme establishing that all the later versions, the modified and the extended ones, are part of the same free programme
a sort of alternative legal system of the copyright version and not a refusal of the legal protection reserved to Software.
an alternative model of handling the copyright system entirely based on the copyright and thanks to it that copyleft can subsist and work.
Using the copyright so as not to deprive the users of their freedom rather so as to protect their freedom.
The core concept of copyleft consists in giving to anyone the right of excuting, copying editing and distributing the programme and its customized versions without permitting to add any further restriction. The users, if they decided to redistribute the modified version, can do it under the same legal system (usually under the same license) in order to guarantee the copyleft regime and the set of freedom ensueing from it.

3. TYPES OF COPYLEFT STRONG COPYLEFT WEAK COPYLEFT
The "strong copyleft" is that one not permitting such dispensation, therefore, once it is modified or implemented a programme under this type of license the redistribution of the developed software must have the same type of license.
VIRAL EFFECT : it propagates itself following the rules of the software it gets in touch with. As a consequence, this implies the impossibility of utilizing components released with a GPL license within a proprietary software unless one has the intention of opening the proprietary software and releasing it with a GPL license
GNU GPL
WEAK COPYLEFT
it refers to the licenses which derivated works will not inherit the copyleft license (it often depends on the way they are issued from it)
weak copyleft is usually utilized for the creation of software libraries so as to permit another programmer to integrate and redistribute them without the necessity of being subordinate to the same copyleft license
GNU LGPL

4. To apply copyleft
Whoever can use the copyleft on condition that he is the real owner of the rights and has the full availability of these rights.
No formality is requested
The owner of the rights must simply choose one among the available licenses and apply it to the software. In order to do that, what is needed is a disclaimer notice declaring what type of license the work is realesed with.