Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is more often known as "Linux", and many users are not aware of the extent of its connection with the GNU Project.
There really is a Linux; it is a kernel, and these people are using it. But you can't use a kernel by itself; a kernel is useful only as part of a whole system. The system in which Linux is typically used is a modified variant of the GNU system--in other words, a Linux-based GNU system.
Many users are not fully aware of the distinction between the kernel, which is Linux, and the whole system, which they also call "Linux". The ambiguous use of the name doesn't promote understanding.
Programmers generally know that Linux is a kernel. But since they have generally heard the whole system called "Linux" as well, they often envisage a history which fits that name. For example, many believe that once Linus Torvalds finished writing the kernel, his friends looked around for other free software, and for no particular reason most everything necessary to make a Unix-like system was already available.
What they found was no accident--it was the GNU system. The available free software added up to a complete system because the GNU Project had been working since 1984 to make one. The GNU Manifesto had set forth the goal of developing a free Unix-like system, called GNU. By the time Linux was written, the system was almost finished.
Most free software projects have the goal of developing a particular program for a particular job. For example, Linus Torvalds set out to write a Unix-like kernel (Linux); Donald Knuth set out to write a text formatter (TeX); Bob Scheifler set out to develop a window system (X Windows). It's natural to measure the contribution of this kind of project by specific programs that came from the project.
If we tried to measure the GNU Project's contribution in this way, what would we conclude? One CD-ROM vendor found that in their "Linux distribution", GNU software was the largest single contingent, around 28% of the total source code, and this included some of the essential major components without which there could be no system. Linux itself was about 3%. So if you were going to pick a name for the system based on who wrote the programs in the system, the most appropriate single choice would be "GNU".
But we don't think that is the right way to consider the question. The GNU Project was not, is not, a project to develop specific software packages. It was not a project to develop a C compiler, although we did. It was not a project to develop a text editor, although we developed one. The GNU Project's aim was to develop a complete free Unix-like system.
Many people have made major contributions to the free software in the system, and they all deserve credit. But the reason it is a system---and not just a collection of useful programs--is because the GNU Project set out to make it one. We wrote the programs that were needed to make a complete free system. We wrote essential but unexciting major components, such as the assembler and linker, because you can't have a system without them. A complete system needs more than just programming tools, so we wrote other components as well, such as the Bourne Again SHell, the PostScript interpreter Ghostscript, and the GNU C library.
By the early 90s we had put together the whole system aside from the kernel (and we were also working on a kernel, the GNU Hurd, which runs on top of Mach). Developing this kernel has been a lot harder than we expected, and we are still working on finishing it.
Fortunately, you don't have to wait for it, because Linux is working now. When Linus Torvalds wrote Linux, he filled the last major gap. People could then put Linux together with the GNU system to make a complete free system: a Linux-based GNU system (or GNU/Linux system, for short).
Putting them together sounds simple, but it was not a trivial job. The GNU C library (called glibc for short) needed substantial changes. Integrating a complete system as a distribution that would work "out of the box" was a big job, too. It required addressing the issue of how to install and boot the system--a problem we had not tackled, because we hadn't yet reached that point. The people who developed the various system distributions made a substantial contribution.
The GNU Project supports GNU/Linux systems as well as the GNU system--even with funds. We funded the rewriting of the Linux-related extensions to the GNU C library, so that now they are well integrated, and the newest GNU/Linux systems use the current library release with no changes. We also funded an early stage of the development of Debian GNU/Linux.
We use Linux-based GNU systems today for most of our work, and we hope you use them too. But please don't confuse the public by using the name "Linux" ambiguously. Linux is the kernel, one of the essential major components of the system. The system as a whole is more or less the GNU system.