Linux is a freely available, UNIX-like operating system.
It was developed by Linus Torvalds in 1991 and since then has been a joined effort of many other developers around the world.
Linux in narrower sense is just the kernel and only together with other programs (mostly GNU software) it forms a complete operating system.
Since Linux, as most GNU software and many other programs under Linux (e.g. VDR) is licensed under the GPL (General Public License), its source code is freely available and can be changed and distributed.