KDE (short for K Desktop Environment) isn't just one of the many Linux desktop environments. Linux KDE is one of the two major Linux desktop managers (the other being Gnome) and it has its hardcore fans. In a sense, Linux KDE looks more like Windows and maybe this is why many people, especially Linux newbies, find it easier to use Linux KDE than Linux Gnome or any of the other Linux desktop managers.
However, it is not true to say that Linux KDE is popular only because it is Windows-like. Gnome has the standard UNIX-appeal and in terms of beauty, definitely KDE has the lead. I don't mean that Gnome is ugly – not at all but it certainly doesn't have the visual design extras of KDE. It is true that beauty is only skin deep, but when you interact with a GUI all day, you definitely would like it more if it is more beautiful.
Visual appeal is only one of the differences between KDE, Gnome, and the other Linux GUIs. The more substantial difference is between the applications included in Linux KDE and the other desktop environments. Usually there are applications, which are part only of one of the desktop managers, so if you need a particular application, you'd better stick to the desktop manager that has it by default.
However, as a rule (especially if the application is popular), there is a way to run a KDE application on Gnome and vice versa, so actually this isn't that much of a difference either. Additionally, the choice of applications depends on the distro– even the distros that use KDE don't necessarily include the same KDE applications. Even if you get a distro that doesn't come with Linux KDE, you can still install KDE later. Some distros come with both Gnome and KDE, or in the case of Ubuntu, they come with Gnome by default but there is also a KDE version (called Kubuntu) ready for you to download.