What is Linux ?
Linux is a free (as in freedom) kernel licensed under the GPL and originally developed by Linus Torvalds in the early 1990s. "Linux" is often used to refer to a GNU/Linux system in it's whole. We prefer and suggest however to use "GNU/Linux" when talking about an operating system based on the Linux kernel.
Nonetheless Linux is very important, cause it's one of the major and most successful GNU projects. Due to Linus' success story the free software movement has evolved enormously in the last years and contributed to create a free IT environment. As a matter of fact GNU/Linux become a natural opponent of Microsoft Windows, which on an economic point of view is really good, cause it tends to balance the monpolistic OS market, at least on the PC level.