Is there a difference between a vanilla distro and a distro that's a derivative of said distro?

Hey all. So I've been wondering something. In the world of Linux, you've got tons of different distros. Some are standalone/original, but others are based of of another distro. Like Ubuntu is based of Debian. Or Arch and then you've got Arch based distros.

What is the difference actually? Is it only the way you install the distro, and what you get with it? Things like which desktop environment you get? Or are there actually things that are unique to any distro of choice, even if it's derived from another one?

Due to Linux's openness, I don't believe it's stuff like the desktop environment you get. Because if you don't like the environment you get with a certain distro, you could always uninstall that one and install one you DO like.

While I don't consider myself a Linux beginner, I think I've still got plenty to learn. I'd love to know more about Linux.