Linux

Linux is a freely available, UNIX like operating system.

It was developed by Linus Torvalds 1991 and since then together with many other developer around the world

Linux in narrower sense is just the kernel and only together with other programs (mostly GNU software) it builds a complete operation system.

Since Linux, as most GNU software and many other programs under Linux (e.g. VDR) is licensed under the GPL (General Public License), it's source code is freely available and can be changed and distributed.