GRASS is free software, you can redistribute it and/or modify it 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.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. See the GNU General Public License for more details at http://www.gnu.org or get the PDF GPL document here.
If you are not sure about the differences between public domain software, Free Software and proprietary products, have a look at this graph describing Categories of Free and Non-Free Software.
Questions regarding details of this policy can be directed to this email address for a prompt response.
See also the GPL FAQ - Frequently Asked Questions about the GNU GPL.