How is Linux Licensed?
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. 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.