what is GNU/linux?

GNU/Linux is an operating system & kernel which comes together to create what is commonly (and incorrectly) known as "Linux" alone.
Linux is the kernel created by Linus Torvalds, and GNU is an entire unix-like (GNU stands for GNU's Not Unix)operating system created by (primarily) Richard Stallman and hundreds of volunteers around the world.
Take a look at http://www.gnu.org and http://www.linux.org

What makes GNU/Linux even more correct is that the GNU system runs on other kernels, e.g. the Hurd (not really a kernel, but a kernel [gnumach] and a set of servers...but let's just call it a kernel for simplicity). When using the Hurd and Linux as the kernel of the GNU system you have basically the same system. All of your favorite commands will work and most programs will compile for either a GNU system using the Hurd or Linux. The only difference is that files are in slightly different places (the Hurd no longer has /usr, everything goes in / [this isn't a problem because of shadowfs and other neat things]), but they are still the GNU system. You can even run GNU on top of e.g. Solaris and get a system that feels the same (if you replaced the solaris tools with the GNU versions; I don't think too many people would do that--the point is that you _could_ if you wanted to). I think that more people will be able to see that "Linux" is really GNU with Linux as its kernel when Debian/Hurd is released with Debian 3.1...

You can even run GNU on Windows via Cygwin! So by saying that it's just Linux not GNU/Linux implies that you are able to run the Linux kernel on a windows platform, which is a completely idiotic idea.