KDE is one of the several GUI's for linux. Technically, they are all different window managers for the X-Window system (the GUI itself). Some of them, such as KDE and GNOME, come with an assortment of programs for various tasks. For example, KDE comes with KOffice, an office suite. Others are more minimal, and only manage windows, and nothing else. One such window manager, my personal favorite, is Fluxbox.
The various window managers are reasonably stable. Sometimes you may encounter a crash or a program that freezes. In these cases, press Alt Ctrl Backspace will either exit or restart X-Windows, and you can start it up again. Its a lot faster than rebooting in Windows, however.