GNU General Public Licence. It's a licence from the FSF -- Free Software Foundation. Any software covered under the GNU GPL is freely distributable and must include source code, which anyone is free to modify or use in their own projects. Any modification or new release of the software or software using the code has to be released under the GPL. There are many other licences as well, such as the BSD licence, Artistic Public licence, OSS licence and many others.
This is different from Public Domain in that the author keeps credit and ownership of his software, and is free to change the licence of any subsequent release.
[ July 21, 2003: Message edited by: Laukev7 ]