A fake autotile is basically what it says, it's a single ''normal'' tile that pretends to be an autotile, it causes all surrounding autotiles to connect to it like it's one of their own. It has its niche uses like when you're trying to get certain tiles to blend together.
A cliff autotile is just that, a cliff. It uses the 4x4 tile preset from RPG Maker to create cliffs, walls and other such things in an orderly fashion. It has its flaws with having several layers of cliffs next to eachother, but for straight lines it works fine out of the box.