GNU/Linux refers to the combination of the GNU tools with the Linux kernel to form a complete operating system. The term was proposed by Richard Stallman and is used by the Debian project (among others) to acknowledge the contributions of GNU software that make it possible to build an operating system around the Linux kernel. Richard Stallman urges users to prefer the term "GNU/Linux" when referring to the whole operating system (and not just the kernel). While other development teams have contributed important aspects of a complete system, the GNU tools are still at the heart of all Linux distributions.
The use of 'GNU/Linux' as opposed to 'Linux' indicates a political preference, not a difference in functionality.