Copyleft

It is not forbidden to appropriate public domain software, that is to sell users something which belonged to them in the first place.

Copyleft imposes that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it.

Copyleft uses copyright law to guarante that every user has freedom.

The most widespread copyleft software license is the GNU General Public License (GPL) of the Free Software Foundation.