This article is from the Frequently Asked Questions for Linux, the Free/Open Source UNIX-like operating system kernel that runs on many modern computer systems. Maintained by David C. Merrill with numerous contributions by others. (v1.0).
A: Linus has placed the Linux kernel under the GNU General Public License, which basically means that you may freely copy, change, and distribute it, but you may not impose any restrictions on further distribution, and you must make the source code available.
This is not the same as Public Domain. See the Copyright FAQ, ftp:// rtfm.mit.edu/pub/usenet/news.answers/law/copyright, for details.
Full details are in the file COPYING in the Linux kernel sources (probably in /usr/src/linux on your system). There is a FAQ for the GPL at: http:// www.gnu.org/copyleft/gpl-faq.html.
The licenses of the utilities and programs which come with the installations vary. Much of the code is from the GNU Project at the Free Software Foundation, and is also under the GPL. Some other major programs often included in Linux distributions are under a BSD license and other similar licenses.
Note that discussion about the merits or otherwise of the GPL should be posted to the news group gnu.misc.discuss, and not to the news:comp.os.linux hierarchy.
For legal questions, refer to the answer: Where Are Linux Legal Issues Discussed?.