If GNU/Linux does not seem to be like one operating system like Windows or OS/X, that is because it is not. For one, Linux is a kernel, and GNU is a tool-set. These can be (as often are) combined to make a working Operating System, but not always. GNU can use other types of kernel (e.g. GNU Hurd), and the Linux kernel can be used by other types of systems (e.g. Android). So GNU/Linux together makes a certain type of Operating System, but depending on who makes the system, these can be called different names.
Enter the distributions
Of course these different “makes”, or flavours of GNU/Linux have a name, they are called distributions, (or distros in short). Whoever takes the Linux kernel and applies a set of tools like GNU (and probably some others), essentially creates a new distribution. There is another way as well, like e.g. take an existing distribution, change stuff in it and re-brand it with a new name, that is also a distribution (a derivative of the one that you’ve originally used to make it). Confused yet? Don’t worry, there is more…
What sets major distributions apart is the amount of work people invest in them and the amount of support you get from the maintainers. There are a few “original” distros as well, like Debian, RedHat, OpenSuse, Gentoo, Arch, etc., which were not based on others, but built from the ground up. From these plenty of derivative distributions are made. For example, the probably most well-known Linux distro Ubuntu is based on Debian.
Linux Mint, another favourite among users, is based on Ubuntu, but because Ubuntu is based on Debian, Mint is still part of the “Debian ecosystem”. If you really like to complicate things (even further), you could make a distro, based on Linux Mint. This would be then based on Ubuntu, which is Based on Debian. So your distro would be based on Mint, Ubuntu and Debian separately, and all of them at the same time. It would have inherited from all three, like a great-grandchild... :) Jokes aside, that is exactly what people do sometimes. They base a new distro, on another one, that is four-to-five times removed from the original base. If that does not sound sensible, that is because it's not.
Distros that are based on others, inherit most features of the base distro and usually add a set of features of their own. Otherwise, it would make no sense to make a new distro, would it? Anyway, to call them Linuces (after the fashion UNIX-like systems are called Unices), would not be quite correct, as it would still be the same (the one and only) Linux kernel, not many Linux-like kernels, whatsoever, although there will be modified versions of it, most likely).
KDE, Gnome and the rest
Besides having different distros, each distro might offer different “flavours”. This is basically the sort of Desktop Environment (or DE in short), they offer. In the old days, you would get a nice big and difficult installer, which let you choose your DE, or even multiple DEs (Debian still follows this fashion).
Derivative systems tend to (over)simplify things, and make the choice for you. A flavour comes with a specific DE like Ubuntu used to have its own DE, called Unity (they're using GNOME now). You can also get Kubuntu (which is Ubuntu with KDE desktop), or there was (in the Unity days) Ubuntu Gnome (which was Ubuntu with Gnome desktop), or Lubuntu with the LXDE desktop, etc. (Why Ubuntu Gnome was not called Gubuntu will forever remain a mystery…)
Debian GNU/Linux will not offer flavours, but lets you choose the environment you’d like to use at install time. You can choose more than one, and select the one you’d like to use at login time.