Traditionally the very diverse array of beings [which inhabit the cosmos] is divided into five gati or “destinations of rebirth” (DN 33). From lowest to highest, these are:
The niraya beings which live in great misery, in a world of fire and cruelty,
The peta beings which exist as wretched shades,
The humans, and
The devas, beings of splendid subtle forms who enjoy long lives of bliss (in this fivefold scheme this category is taken to include the brahmās as well).
We shall see that this simple scheme is inadequate for anything more than the roughest outline. In practice, there are great variations within each category and there are numerous individual beings and in fact whole classes of beings which are difficult or even impossible to pigeon hole within one of the five gati. This was already apparent in ancient times, and later schools of Buddhism expanded this scheme to include a sixth gati for the asuras, ferocious beings endlessly at war with the devas.
A more useful and inclusive method of analysis is provided by the Abhidhamma, which divides all reality into four planes (bhūmī): the sensual-desire plane (kāmābhūmi), the plane of form (rūpabhūmi), the plane of the formless (arupabhūmi) and the supramundane (lokuttara, lit. “beyond the world.”) The last is nibbāna, the unconditioned, and is not a part of the cosmos at all. The first three apply to levels of consciousness as well as realms of existence.
Kāmabhūmi is the realm of sense desire. The psyche of beings in this realm is dominated by the five physical senses and their motivation is normally to acquire pleasant sense experiences and avoid unpleasant ones. This is the most diverse and populous of the realms and includes beings in niraya, animals, petas, asuras, yakkhas, humans and all the various levels of devas below the brahmā world.
Rūpabhūmi is the realm of form. This is the abode of the various levels of brahmā beings. These beings have transcended sensuality; they have no gender and they take no coarse food, instead “feeding on bliss.” There are altogether sixteen levels within this realm including the Suddhāvāsa, the “Pure Abodes” where those who have reached the third stage of awakening are reborn.
Arūpabhūmi is the formless realm. This is the realm of beings that have no physical bodies but are pure mind. It cannot therefore be located in space, spatial location being a physical property. There are four levels within this realm.
The entire range of beings is often presented in late sources as a list of 31 realms.
This reflection by Ajahn Punnadhammo is from the book, The Buddhist Cosmos. Page 17 of the pdf download link here features an in-depth table of the above.