Linux is an operating system. It is software that controls how hardware components in the computer operate.
Unlike Microsoft Windows, Linux is under the General Public License.
"GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users." http://www.gnu.org/copyleft/gpl.html
Which means you can download Linux and use it on your computer free of charge. See www.linuxiso.org