How to (not) use GPL Software
Ideas and Goals of the GNU GPL

Free Software
Software that has fundamental freedoms:
to use it for any purpose
to "help your neighbour" (i.e. make copies)
to study it's functionality (reading source code)
to fix it myself (make modifications and run them)

Copyleft
Is the legal idea to 
exercising copyright to grant the above freedoms
assure that nobody can take away the freedom

The GNU General Public License
Is a legal instrument to apply they copyleft idea on software