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@stefan Some people might hate me for saying that, but I think that "the UI that Ubuntu uses" is a pretty decent explanation.
It's a gross simplification. As GNOME encompasses includes quite a bit more than just the UI itself (including system components, like how to draw windows on your screen, or an ecosystem of applications), but for the layman that's the main thing you need to know.
And on that same path. KDE is a different UI, that looks more like Windows than GNOME does.
I love that explaination. Because it is that what matters for the end user.
Along with the fact that the Linux you use is made of many tiny programs that work together and are interchangeabl, maybe. What is a simplification as well.