Linux is a free Unix-type operating system originally created by Linus Torvalds with the assistance of developers around the world. Developed under the GNU General Public License , the source code for Linux is freely available to everyone.
And the little bit longer version: Linus Torvald is the 'head creator' of the Linux kernel. At the same time as he was creating the kernel, GNU (or what is now GNU) was creating another OS. GNU had created a lot of the programs etc that an OS needs, but they did not have an kernel. So these to projects 'merged' and became what we today call Linux.