-dev contains the headers, that ones pretty easy.

The number is a version for when they provide more than one that can be installed at once (excepting where it's part of the library name, presumably like xcb1).

I.e. at some point you could probably install libgtk-3-0 and libgtk-2-$something at the same time. They likely leave it that way when they get rid of libgtk-2 so that existing tutorials that reference libgtk-3-0 don't fail because the package is now libgtk.

The libwayland-server0 one does get me, I don't know why there's a 0 at the end. I do know that in /var/lib there is often the same library with various .$number endings, but I've never looked into what that accomplishes.