The cluster is currently the one containing most of what is described on the wiki. It's made to bring something that does not exist in our world, aka fantasy and high sci-fi things.
Planes
Planes represent blobs of relatively pure kinds of energy. There also can be subplanes that make up the planes and other subplanes and the further you deconstruct them the cleaner kinds of energy you will get, in the getting back to the Chaos though.
Planes can interact with each other if they intersect.
Matera
The material plane. This is the most obvious and known to us plane that does not make it any simpler, even now we try to understand the laws of it. Our own Dimension would consist entirely of it (if you are as sceptical as the author is). It will be a very frequent guest in most of the worlds as it is one easy to build a Dimension upon on.
Psyche
The plane that contains all the existing psyches and their energy. In the intersections with the Material Plane mostly can be find the psycheful creatures. Just to be precise: you don't need a psyche to be sentient or a living creature at all. In fact a psyche is a connection to the plane. The plane itself was created by race of mysterious creatures presumably to collect data and knowledge and somehow share it.
Arcane
The presumably artificially created by race of mysterious creatures hidden plane. It contains nearly infinite amounts of very flexible energy tightly bound to many Planes, mostly the Material Plane. To see how it is used to make magic read the article.
Unknown
There are way more planes than is represented here and they are only about to be known in any way.
Dimensions
Finally we are here.
Dimensions are the worlds where will happen all the interesting events. Using the word "dimension" is supposed to be more strict than the "world" word because Monada itself is a world. From now and further on the "dimension" word will represent a specific thing that is inside the Monada.
Dimensions can consist of one or many Planes, mostly at areas of their intersection.
Dimensions move chaotically together with the Planes that make them up. Sometimes this moving can make two or more Dimensions to intersect and this is how walking between Dimensions can happen.