Basicly its what they call a "desktop environment" its the whole design/layout and function of the bits you see.
Gnome is used in Ubuntu and theres a varient of Ubuntu called Kubuntu which uses KDE.
I prefer Gnome where as entwisi prefers KDE, its really just personal preference.
KDE looks a tiny bit more like windows by default but theres really nothing in it.
Certain applications are built for certain desktop environments but they all work on either one so its not important which one you use
edit: Thats kinda confusing without real examples.. it'll make more sense to you when you start using Ubuntu and learn linux a bit more
