Comment 51 for bug 1652282

I'm only recently familiar with the Mutter design, but...

Mutter is the graphics platform on which gnome-shell is built. Mutter supports running under a Xorg server, or none at all on bare metal (Wayland mode). So in order to provide a seamless consistent interface in the latter case it spawns Xwayland, which allows X apps to work with Wayland even when Mutter's not running on Xorg. In this way, any shell built on Mutter has the option to use Xorg or Wayland, and if using Wayland can still transparently run X apps.

"Clutter" is the graphics toolkit on which Mutter is built. "Cogl" is the graphics toolkit on which Clutter is built. Cogl uses OpenGL directly. Yes, that's probably too many layers but that's how things evolved.