A Small Page from History
GNU
The GNU project was started by RMS. RMS was appalled by the decline of the MIT Hacker Culture and decided that if things were to progress in the same way, software freedom would have been a thing of the past. So he decided to write the GNU operating system. Read this page for more info.
Linux
The Linux Project was in fact a college level project of Linus Tolvards. Please note, that Linux is a mere kernel. A kernel is one part of an operating system(An integral part though).
Why GNU/Linux
Linus released his kernel at around the same time the GNU coders were done writing the other replacements to proprietary
software. So they used the Linux kernel, since it was open source too and compliant with the then free software philosophy.
So it is a whole bunch of software from the GNU project and just 1 from the Linux project.
Is it not common sense to call it GNU/Linux. People seem to know about the Linux OS. which is in fact a misnomer. Linux is not an OS. GNU/Linux is.
A footnote
There are projects like GNU/Hurd which uses the Mach kernel and the GNU Software set, GNU/NetBSD, GNU/FreeBSD, Nexenta, Schillix etc (GNU/OpenSolaris), which use the respective kernels. There is also the Tunes OS without a kernel
So GNU can live without Linux, but Linux cannot survive without GNU!!!
Long live GNU!!!
