Different display servers. Tl;dr is that Wayland is newer and fully supported while X11 is getting depreciated. Wayland is pretty much feature complete outside the flickering NVIDIA bugs
Compositors, basically what renders your windows for you.
X11 is 40-ish years old and the codebase is a mess. It's been put into "maintainence" (Basically just gets fixes, no new updates) This has led people into thinking X11 is way more stable than it really is, because it's codebase has basically been untouched for a decade.
Wayland is the replacement to X11, and is finally in it's primetime after over a decade of development. It's codebase is a lot sleeker, it is a LOT more secure and is what developers are targeting for new features (e.g. HDR). Wayland is controversial because it still has a few issues and it's more secure approach has some usability trade-offs which are slowly being solved as time goes on, as well as the whole Nvidia issue.