1. The realm of the hell-dwellers is the lowest of the realms. Those who willfully commit the ten evil acts are reborn as hell-dwellers as the fruition of their deeds. Likewise, those who commit one of the five grave offences (killing one’s father, killing one’s mother, killing an arhat, injuring the Buddha, causing a schism in the Sangha) will immediately be reborn in the lowest hell in their next life. The hot hells and cold hells (perhaps for the passionately unrestrained and the cold-hearted respectively) are the abode of those so consumed with hatred, bitterness, and despair that their only wish is to destroy themselves and others out of spite and the desire to end their miserable existence.
2. The realm of the hungry ghosts is only slightly better. Those who commit the ten evil acts primarily motivated by selfish craving are reborn as hungry ghosts as the fruition of their deeds. Hungry ghosts are said to have large mouths and bellies, but only tiny throats. Hungry ghosts can never be satisfied and are consumed by craving. This is the state of those who suffer from addictions that control and dominate their lives. These addictions can be to drugs, alcohol, sex, gambling, power, work, entertainment, or even religion.
3. The realm of animals is the state of cunning, primitive aggression, and instinctive desires. Those who unthinkingly commit the ten evil acts are reborn as animals. It is a state of mind that does not look beyond immediate gratification and pays no heed to consequences or long-term benefit. Here, pleasure and pain reign supreme over reason amid the brute struggle for survival as the strong prey upon the weak. Though not as inherently painful as the two previous worlds, those who are in this state will inevitably meet with frustration and confusion, if not outright pain and suffering.
4. The realm of the fighting demons is the realm of arrogant demons who are obsessed with issues of status and power and whose ambition is to overthrow the gods of heaven. Those who follow the five precepts or even the five constant virtues (of Confucianism) but with ulterior motives and in a spirit of hypocrisy and self-righteousness are reborn as fighting demons. Those in this state are full of pride and arrogance and are extremely competitive and envious. They can never rest or feel secure because they must constantly strive to maintain and improve their position and prestige, no matter how well off they may actually be.
5. The realm of humanity is, of course, the realm we are most familiar with. Those who follow the five constant virtues or who take refuge in the Three Treasures (Buddha, Dharma, and Sangha) and follow the five precepts are able to be reborn as humans. In the human world, suffering is recognized for what it is, and morality and reason are called upon to improve the human condition. At this point, civilized life can truly begin. The human state is considered a very fortunate one, because the suffering and striving of the previous four realms does not overcome reason, nor is there the complacency brought about by the pleasures of the heavens. From this realm of humanity, one can find the opportunity to encounter the Three Treasures, take up the teachings, put them into practice, and attain liberation.
6. The realm of the heavenly beings is where the gods make their abode. Those who take refuge in the Three Treasures, follow the ten good acts, and give generously to worthy people and causes are able to be reborn in the heavens of desire. Those who follow the ten good acts and also go on to cultivate states of meditative absorption to overcome all inner disturbance and negativity are able to be reborn in the more refined heavens of form or formlessness, which correspond to the states of concentration they attained. The heavens are temporary (though long lasting) realms of spiritual bliss of increasing subtlety and refinement.
7. The realm of the śrāvakas is the first of the four noble states. Śrāvakas are those who hear the teachings of the Buddha, specifically the four noble truths, and put them into practice by becoming monks or nuns and taking up a life of strict discipline and rigorous contemplative practices in order to awaken to the fact that all conditioned things are ultimately unsatisfactory, impermanent, selfless, and empty of any fixed unchanging essence. This awakening is their attainment of nirvärpa. Nirväna, for them, is the elimination of the fetters of greed, hatred, and false views that bind them to rebirth among the six lower realms. Upon attaining nirvāṇa, śrāvakas are known as arhats (lit. worthy ones), who are worthy to receive offerings. Though this is called a realm, arhats do not exist in a separate realm apart from the world of humanity. Arhats are invariably human beings who attain nirvāṇa and upon their deaths (which is called final nirvāṇa) they are no longer to be found anywhere, much like a fire that has been extinguished.
8. The realm of the pratyekabuddhas is the second of the four noble states. Pratyekabuddhas are the ascetics and hermits who live in a time and place where Buddhism is unknown but attain liberation by contemplating causes and conditions and awakening to the unsatisfactory, impermanent, selfless, and empty nature of all conditioned phenomena. Unlike buddhas, they do not afterwards try to teach others how to awaken. Like the arhats they are invariably human beings who have eliminated greed, hatred, and false views. They too have realized nirvāṇa and are no longer bound to be reborn among the six lower realms. Sometimes pratyekabuddhas are not viewed as hermits who awaken on their own apart from Buddhism. Sometimes they are thought to be those Buddhist practitioners who live as forest-hermits and attain nirvāṇa by contemplating the twelve-fold chain of dependent origination taught by the Buddha to provide a deeper understanding of causes and conditions. In this case they are known as “cause-knowers” (J. engaku).
9. The realm of the bodhisattvas is the third of the four noble states. Bodhisattvas are those who aspire to attain buddhahood so that they too can lead all sentient beings to liberation. To do this they make vows, for instance the four great vows of the bodhisattva (to save all being, quench all defilements, know all the teachings, and attain the path to buddhahood), and take up the practice of the six perfections (of generosity, morality, patience, energy, meditative absorption, and wisdom). Bodhisattvas sometimes reside in the pure lands of the buddhas of the ten directions but can just as often be found taking rebirth among those in the six worlds in accordance with their vows so that they can help sentient beings, cultivate wisdom, and accrue the merit needed to attain buddhahood.
10. The realm of the buddhas is the fourth of the four noble states. Buddhahood is the state characterized by purity, bliss, eternity, and self (or authenticity). With perfect wisdom and great compassion, the buddhas spontaneously and unselfconsciously respond to the spiritual needs of all sentient beings. They reside in the pure lands but also appear in the world of humanity as a person who attains buddhahood, teaches the Dharma, and establishes the Sangha.

Each of these ten realms contains the causes and conditions of all ten within themselves. This means that any of the ten contains the potential to manifest any of the others. This is the mutual possession of the ten realms. One might say that the lower realms contain the higher realms as seeds of their future growth and maturity, while the higher realms embrace the lower realms insofar as they are perspectives that have been outgrown and yet assimilated. These are not ten separate realms lined up alongside each other, but rather ten different subjective modes of the interplay of causes and conditions. This means that if you change the causes and conditions you can also change the kind of realm that is or will be experienced. This also means that the realm of buddhahood is accessible to all the other realms and conversely the realm of buddhahood is able to compassionately interact with the lower realms.