This software is distributed under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

As of writing, this essentially boils down to two possible licenses, GPL version 2 and version 3.  Copies of these licenses are available in files GPL-2 and GPL-3 in the sources of this package, as well as at

http://www.r-project.org/Licenses/

They can also be viewed from within R using 

RShowDoc("GPL-2")
RShowDoc("GPL-3")