left vs right

The open source movement needs protection of the programmer's and users' freedom from proprietary licenses.

from the free software foundation's site:
Proprietary software developpers use copyright to take away the user's freedom; we use copyright to uarantee their freedom. That's why we reverse the name, changing "copyright" into "copyleft".

What is copyleft?:
"Copyleft is a general method for making a program free software and requiring all modified and extended versions of the program to be free software as well."

Copyleft is enforced mostly by the General Public License (other licenses are variations on it), distributed along with the source code stating the work to be copyrighted under its terms: that is that the source code can be used and modified, (even charged for) but that the resulting source code be made freely available. Thus, free here doesn't mean free of cost but free as in 'information wants to be free'.