Many people always mentioning their distribution as Linux, such as: "I'm using Linux" "My Linux is Redhat/ubuntu ..." But, it is a wrong way to address that great operating system as Linux alone. Linux is a kernel. Well, let's put it in a simple description. It is a library that provide mechanisms to arrange system resource and do something with your machine. It provide service such as running applications concurently, accessing memory, accessing devices like printer etc. But, in order to run the library, you must provide a framework that accessing that library. Luckily for FOSS, Stallman and the crew have released series of tools that running on top of Linux kernel. Those tools (GNU Libc, GNU BASH, GNU C Compiler, etc) is provided as a whole-set-solution framework called GNU (GNU Not Unix). So, if you would mention that system, which runs GNU framework in a top of Linux kernel, you should mention it as GNU/Linux.