First of all, Linux varies very widely from distribution and from people to people; there is a lot of choice in what components people put in their system. A large fraction of people don't install a GUI at all. All components of the GUI are optional and modular.

The basis of the GUI is X (aka the X Window System), which is a protocol that allows interfaces to be built on top of it. To use it, one usually have one of many simple window managers or a sophisticated desktop environment (like GNOME or KDE). How these components appear depends on what the user sets and differs widely (not standardized like Windows).

comes as close to saying it all as you can in 3 words.

Because of the nature of Linux, the diversity verging on chaos, it is hard to be specific & brief.

If you need more info, how about telling us why you ask & what you have already read.

And if you tell us when your homework has to be handed in, we can try and give it in timely fashion

Incidentally, do you include the framebuffer in your question about the GUI? Are you interested in the history (such as Mosaic) or just the present (GTK/QT)? etc.