The first website I found is titled Perspective Drawing. It talks about the different angles when you are drawing something in perspective, like when you are drawing, it uses geometry. Every time you are drawing in perspective, you need a vanishing point and the plane of drawing. You need these factors because it adds realism.
The website gives pictures backing up what it is saying, so you can understand the topic further. It also gives reference to actual paintings using perspective if you want to see an example.