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).