Linux was born in 1991. Initially, it was just a kernel, developed by Linus Torvalds. Nowadays people commonly refer to a GNU/Linux system as just "Linux". As the full name of the OS says, GNU/Linux is a combination of the Linux software and GNU software components. GNU, an acronym for GNU’s not Unix, started out as a project in the early nineties to develop a free Unix-like OS. Now, lots of the applications on a Linux system are provided by the GNU project. Examples are the GNU C compiler (GCC) and the bash shell on your Linux box. More info on GNU/Linux can be found on Wikipedia.