Many people feel that Linux should really be called GNU/Linux. This is because Linux is only the kernel, not the applications that run on it. Most of the basic command line utilities were written by the Free Software Foundation while developing their GNU operating system. Among those utilities are some of the most basic commands like cp, mv lsof, and dd.
In a nutshell, what happened was, the FSF started developing GNU by writing things like compliers, C libraries, and basic command line utilities before the kernel. Linus Torvalds, started Linux by writing the Linux kernel first and using applications written for GNU.
I do not feel that this is the proper forum to debate what name people should use when referring to Linux. I mention it here, because I feel it is important to understand the relationship between GNU and Linux, and to also explain why some Linux is sometimes referred to as GNU/Linux. The document will be simply referring to it as Linux.
GNU's side of the issue is discussed on their website:
The relationship - http://www.gnu.org/gnu/linux-and-gnu.html
Why Linux should be GNU/Linux - http://www.gnu.org/gnu/why-gnu-linux.html
GNU/Linux FAQ's - http://www.gnu.org/gnu/gnu-linux-faq.html
Here are some Alternate views: