Software Licenses: A software license is a license that grants permission to do things with software. The license can be used to grant permissions to do things which are not granted by copyright. The license can also be used to deny users the right to do things to software to a much larger degree than those granted by copyright.
Linux: Linux is a free Unix-type operating system. Developed under the GNU General Public License, the source code for Linux is freely available to everyone.
Free Software (FS): A term denoting software which fulfills the four freedoms, a set of standards set by the Free Software Foundation. See more http://www.gnu.org/philosophy/free-sw.html .
Copyleft: Copyleft is a general method for making a program or other work free, and requiring all modified and extended versions of the program to be free as well. Copyleft says that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. Copyleft guarantees that every user has freedom.