Originally, it was meant to stand for "Kool." It now does not stand for anything, thus making KDE stand for "K Desktop Environment."
K Desktop Environment
Kubuntu is Ubuntu packaged with the KDE Plasma desktop environment. KDE is known for its fancy looks, bundled with the popularity and support of Ubuntu.
KDE stands for K Desktop Environment (the K originally stood for Kool) and is useful on Linux systems to provide a Windows-like appearance to the user.
Everything will still work. However gnome applications do not interact with the desktop environment of KDE as well as they would with gnome. The differences will be mainly superficial. You can always remove the gnome applications later and replace them with kde ones if you want.
Desktop environments.
There are many desktop environments in linux. Most prominent would be: KDE Trinity - KDE fork (response to KDE 4.0 dissatisfaction) Gnome MATE - Gnome fork (response to Gnome 3.0 dissatisfaction) Cinnamon - Gnome fork (response to Gnome 3.0 dissatisfaction) Unity - Ubuntu's desktop environment (built on top of Gnome 3.0) XFCE LXDE There are many more. Also, there are window managers that have comparable functionality (ex. Enlightenment)
There is no "default" in the sense that Fedora provides only one out of the box, or even one at all. The main "Desktop Edition" disc uses GNOME, but the project also provides discs for KDE, LXDE, and XFCE, as well as a CLI-only installation.
desktop
there are too many Gnome, unity, kde, ....
Xfce LXDE Fluxbox Blackbox Openbox JWM Mezzo WindowMaker GNUStep
KDE and GNOME are the result of two different attempts to create a free and open-source desktop environment for Linux. KDE was created first. KDE used a widget toolkit known as "Qt", which was not open-source back then. Since some users wanted one that was completely open-source, even the toolkit, the GNOME project was created. Many users still liked KDE, though. Those who did not want to switch stuck with KDE. Those who insisted on free software chose GNOME. Later, the Qt toolkit was open-sourced, which removed much of the incentive for GNOME to exist, but enough users at that point like GNOME better, so development has continued.
i assume by gui, i assume you mean the window manager. gnome and kde are popular. i personally like to use xfce, the "cholesterol free desktop environment".