When you open Unity for the first time, the 'default' layout of the most common editor windows will appear. Before discussing their purpose and function, let's first navigate to where you can reset the layout, should you lose or misplace a window.
In the Unity menu, select Window > Layouts.
Here you will find five presets of how to arrange your Unity Editor windows. The 'Default' version is the one appearing when you open it for the first time. As such, if you ever lose or misplace a window, you can navigate here and select 'Default'.
You will now know how to get back to the starting layout, so you can proceed to learn about each window.