Understanding the General Public License
The Linux kernel is freely distributable under the General Public License (GPL) developed by the Free Software Foundation. (For the full text of the GPL, see Appendix B in the book. This license is also called The GNU General Public License.)
It is the intention of the GPL to promote free distribution and open development of software. The Free Software Foundation is at pains to point out that free under the GPL does not mean the same thing as no cost. In fact, you are allowed to sell software licensed under the GPL, but you cannot place restrictions on others reselling what you have sold to them. As the Linux evangelists say, think free speech, not free beer.
Hence, the word free is really being used in a political, or visionary, sense. It means liberated from licensing restrictions, and intellectually free. This is in keeping with the vision of the Free Software Foundation: its goal is to create a complete operating system with all necessary applications and utilities that can be distributed without restriction.