Licenses are the core of OS/FS, setting out the relationships between originators and contributors of code, and further developments of programs.
Embody "moral" issues of ownership, ethics and reputation, aswell as pragmatic ideas about how the "best" software is produced.
Extremely contentious, as the exact licenses under which progams are distributed determine whether they can be included in Open Source packages such as Linux and BSD.
Many licenses:
http://www.gnu.org/licenses/license-list.html
http://www.opensource.org/licenses/
Most important :
My friend Toni Prug suggested the following additions to history and defintions:
http://www.cbbrowne.com/info/lsfmandate.html#GPLVSBSDL (licence wars)
Documentation very important in OS/FS practice, and also licensed.
http://www.linuxdoc.org/
GPL never tested in court...
http://www.linuxplanet.com/linuxplanet/reports/2000/2/
very recent case:
http://www.gnu.org/press/2002-02-26-MySQL.html
Plus:
derivities that are not related to software!