Typically copyrights take away freedoms; copyleft preserves
them. It is a legal instrument that requires those who pass
on a program to include the rights to use, modify, and
redistribute the code; the code and the freedoms become
The copyleft used by the GNU Project combines a regular
copyright notice and the "GNU General Public License" (GPL).
The GPL is a copying license which basically says that you
have the aforementioned freedoms. The license is included in
each GNU source code distribution and manual.
See also General Public Virus.