Extracted from the website of Buddhist Vajrayana Charity Funds Association and translated (佛教的有情觀) into English:

The Buddhist View on Sentient Beings

In Sanskrit, Sattva translates to sentient beings, referring to all those creatures that have consciousness and that have life. Their form and way of life are classified into the Six States of Existence. How they are born is classified into Four Births. According to their nature and spirits, they are classified into Three Realms.

(1)	The Six States of Existence
	Karma (behaviour) is created by sentient beings due to confusion arising (affliction). The aggregate of behaviour creates the energy of reincarnation within the Six Paths. From good karma there is birth within the ‘Three Good Paths’ of Heaven, Human and Asura. From bad karma there is birth within the ‘Three Evil Paths’ of Hell-dweller, Hungry Ghost and Animal.
	1.	Heaven: called Deva (Sanskrit), meaning radiant /shining, pure, free from obstacle and most superior. Those born in heaven have bodies that are surrounded by brightness and enjoy more happiness than hardship. If one cultivates the Ten Good Deeds, one will go to Heaven.
	2.	Asura: a Sanskrit word, means ugly. The fortune reward is similar to that of Heaven, but Asuras like to create conflict, their willpower is tantamount to deceit, with arrogance, anger and stubbornness.
	3.	Human: being born as human originates in practising the Five Precepts and performing the Good Deeds. As a bittersweet taste seems bitter (sad) and sweet (happiness) at the same, one has to cultivate the (spiritual ) path (Too much happiness in Heaven, too much hardship in the Three Evil Paths, no time to cultivate.)
	4.	Animal: the new translation is Tiryagyoni (Sanskrit), not being born in the right path of either Human or Heaven. Those in this state brutally kill each other; they suffer from the pain of being swallowed, battered, abused and killed. Those with stupidity and evil deeds are born here.
	5.	Hungry Ghost: Within the category of ghosts the majority of them are hungry ghosts. Hungry Ghosts with some merits live in temples, graveyards of the mountain forests (山林塚廟). Those of lower levels live in impure land, are not able to eat and drink, and always suffer from the agony of hunger and thirst. This State is generated from the acts of stinginess and evil behaviour. Hungry Ghosts see spring water as pus and blood and with this as their diet and they feel the burn of flames.
	6.	Hell-dweller: Living beneath the ground in hardship is equivalent to imprisonment. Those beings that commit extreme evils deeds are born here.
(2)	The Four Births
	1.	Egg Birth (Sanskrit: Andaja 卵生): From egg shell to body formation; such as goose, wild goose, etc.
	2.	Embryo Birth (Sanskrit: Jarayuja 胎生): Also known as abdomen birth, from the maternal foetus to body formation; such as human beings, cows, etc.
	3.	Wet Birth (Sanskrit: Samsvedaja 濕生): Based on humidity from formation to birth; such as moths, worms, mosquitos, etc.
	4.	Metamorphosis Birth (Sanskrit: Upapaduka 化生): By natural changes, not dependent upon the fate of a father and mother, nor reliant on racial reproduction; such as heaven, hell.
(3)	The Three Realms
	1.	The Desire Realm:
		~	Desire means the desire of love referring to lust (the desire between man and woman) and the appetite for food (material desire) which have material entity.
		~	Possessing Five Aggregates (Form, Sensation, Thought, Volition, Consciousness) The Material World.
	2.	The Form Realm:
		~	Even without the two sexual and eating desires, there would still be the remarkably wonderful physical body, country land and palace dwellings.
		~	Form means the materialistic obstacle.
		~	Based on the level of spiritual accomplishment this can be classified into Four Dhyana Heavens/Four Meditation Heavens.
	3.	The Formless Realm:
		~	No physical body, country land nor palace dwellings. Free from material restrictions, pure spirituality.
		~	Based on the quality of achievement of Dhyana (Meditation), this realm is classified into Four States of Emptiness (the Realm of Infinite Space 空無邊處, the Realm of Infinite Consciousness 識無邊處, the Realm of Nothing Whatsoever 無所有處, the Realm of Neither Cognition nor Non-Cognition 非想非非想處).
The Three Realms >>

The Desire Realm	－	1.	Six Desire Heavens		Material Desire
		2.	Human
		3.	Asura
		4.	Animal
		5.	Hungry Ghost
		6.	Hell-dweller
The Form Realm	－	1.	The First Dhyana Heaven		With Material Without Desire
		2.	The Second Dhyana Heaven
		3.	The Third Dhyana Heaven
		4.	The Fourth Dhyana Heaven
The Formless Realm	－	1.	The Realm of Infinite Space		Without Material Spirit Only
		2.	The Realm of Infinite Consciousness
		3.	The Realm of Nothing Whatsoever
		4.	The Realm of Neither Cognition nor Non-Cognition