GPL stands for the "GNU General Public License." It is a license written by the ?FreeSoftwareFoundation. It is generally applied to software, and is intended to keep software release under it freely copyable.
It is often refered to as the "GNU Copyleft".
The text of the license can be found at 
http://www.gnu.org/copyleft/gpl.txt.
The content of this page is Copyright (C) 2000, 2001, 2002 Geoffrey T. Dairiki and
the other authors of the content, whoever they may be.
This is free information and you are welcome redistribute it
under certain conditions; see
http://www.dairiki.org/HammondWiki/opl.html for details.
Absolutely no warrantee is made as to the correctness of
the information on this page.