Just to explain dd, you can install as many window managers as you want and change between them on the log in screen... the 3 main ones are Gnome, KDE and XFCE, but there are more to choose from.

You can use the same apps on each window manager, but certain apps are designed for certain managers and so look better on them... so generally apps beginning with a k, such as Konqueror or Konsole are for use with KDE and those beginning with a G are for use with Gnome, such as Gparted etc...