It's important to understand the key characteristics of these licenses since they're widely used (GPL maybe more than 70% of total free software).
GNU GPL (GNU General Public License[6] ) :
All the 4 freedoms (of course :-))
+ One may not redistribute modified software with changes to the licensing terms : every derivative (improved, modified, mixed, etc.) version must remain under the GPL.
+ One may not mix code under GPL with software under uncompatible license (i.e.: proprietary software)
GNU LGPL (GNU Lesser General Public License[7] )
Main difference with the GPL : allows mix between software protected by the GNU LGPL and proprietary software.
Both copylefted licenses enforce users rights and the software freedom through a tight legal control.
This is the best way to ensure a live and rich long-lasting free software portfolio.