Source:
All About Buddhism ~ For Non-Buddhists and Buddhists
Originally posted by An Eternal Now:
(continued from above)

In Buddhism, it is taught that there are various realms, spheres or
dimensions of existence. There are thirty-one planes of existence
listed, but for our purposes, we are going to utilize a simpler
scheme which enumerates six realms of existence. In general, the
six realms may be divided into two groups, one of which is
relatively fortunate and the other relatively miserable. The first
group includes three of the six realms and they are the realm of
the gods, the realm of the demigods and the realm of human beings.
Rebirth in these fortunate realms is the result of wholesome karma.
The second group includes the three realms that are considered
relatively miserable. They are sometimes called the realms of woe,
and they are the realm of animals, the realm of hungry ghosts and
the realm of hell beings. Rebirth in these states of woe is the
result of unwholesome karma.
Let us look at each of these realms individually and starting from
the realm at the bottom, let us look at the realm of the
hell beings (Niraya). There are various hells in Buddhism,
and they are principally eight hot hells and eight cold hells. In
the hells, beings suffer incalculable and inexpressible pain. It is
said that the suffering experienced as a result of being pierced by
three hundred spears in a single day in this life is only a minute
fraction of the suffering experienced in hell. The cause of rebirth
in hell is continuous, habitual violent actions - habitual killing,
cruelty and so forth, actions that are borne of ill-will. Beings
born in the hells suffer the pain of hell until their unwholesome
karma is exhausted. This is important because we must note that in
Buddhism no one suffers eternal damnation. When their unwholesome
karma is exhausted, beings in hell are reborn in a more fortunate
realm of existence.
The next realm is the realm of the hungry ghosts
(Pretas). Beings in this realm suffer chiefly from hunger
and thirst, and from heat and cold. They are completely bereft of
the objects of their desire. It is said that when the hungry ghosts
perceive a mountain of rice or a river of fresh water, and rush
towards that vision, they find the mountain of rice is only a heap
of pebbles, and the river of fresh water only a ribbon of blue
slate. Similarly, it is said that in the summer even the moon is
hot, while in the winter even the sun is cold for them. The
foremost cause of rebirth as a hungry ghost is avarice and
miserliness borne of greed. As with the hells, the beings in this
realm are not condemned to eternal existence in the form of hungry
ghosts, for when their unwholesome karma is exhausted, they will be
reborn in a higher realm.
In the next realm which is the realm of animals
(Tiryak), the living beings suffer from a variety of
unhappy circumstances. They suffer from the fear and pain that is
the result of constantly killing and eating one another. They
suffer from the depredations of man who kills them for food or for
their hides, horns or teeth. Even if they are not killed, domestic
animals are forced to work for man and are driven on by hooks and
whips. All these are a source of suffering. The principal cause of
rebirth as an animal is ignorance. In other words, the blind,
heedless pursuit of ones animal-like desires, the preoccupation
with eating, sleeping and sexual desire, and the disregard of
developing ones mind to the practice of virtue and so forth lead
one to be reborn as an animal.
Now when I say for instance that habitual killing is the cause of
rebirth in the hells, or that greed is the cause of rebirth in the
realm of the hungry ghosts, or that ignorance is the cause of
rebirth in the realm of animals, it does not mean that a specific
hateful, greedy or ignorant action will result in rebirth amongst
the appropriate class of beings - the hells, the realms of hungry
ghosts or the realm of animals. What it does mean is that there is
a relationship between hatred and rebirth in the hells, and between
greed and rebirth in the realm of hungry ghosts, and between
ignorance and rebirth in the realm of the animals. If unimpeded, if
unbalanced by other virtuous actions, such actions if habitual are
likely to result in rebirth in these three states of woe.
I am going to skip the realm of human beings for the moment and go
on to the realm of demigods (Asuras). The Asuras
are more powerful physically and are more intelligent mentally than
human beings. Yet they suffer because of jealousy and conflict.
Mythologically, it is said that the Asuras and the gods share a
celestial tree. While the gods enjoy the fruits of this celestial
tree, the Asuras are custodians of the roots of the tree. The
Asuras are envious of the gods and constantly attempt to take the
fruits of the tree from the gods. As a result of this, they fight
with the gods, and are defeated by the gods and suffer greatly as a
consequence. Because of this constant jealousy, envy and conflict,
existence amongst the Asuras is unhappy and unfortunate. As with
the other realms, there is a cause of rebirth amongst the demigods.
On the positive side, the cause is generosity. On the negative
side, the causes are anger, envy and jealousy.
The sixth realm, the realm of the gods (Devas) is
the happiest amongst the six realms. As a result of having done
wholesome actions, of having observed the moral precepts and having
practised meditation, living beings are reborn amongst the gods
where they enjoy sensual pleasure or spiritual pleasure, or
tranquillity depending upon the level within the realm of the gods
in which they are born. Nonetheless, the realm of the gods is not
to be desired because the happiness of the gods is impermanent. No
matter how much they may enjoy their existence as a god, when the
force of their karma is exhausted, when the merits of their good
conduct and the power of their experience in meditation are
exhausted, the gods fall from heaven and are reborn in another
realm. At this moment, at the moment of their death, it is said
that the gods suffer even more mental anguish than the physical
pain suffered by beings in the other realms. The negative factor
associated with birth in the realm of the gods is pride.
So here, as you can see, we have an affliction or defilement
associated with the five realms - hell beings, hungry ghosts,
animals, demigods and the gods, and they are ill-will, desire,
ignorance, jealousy and pride. Birth in any of these five realms is
undesirable. Birth in the three lower realms is undesirable for
obvious reasons, because of the intense suffering and because of
the total ignorance of the beings who inhabit these realms. Even
rebirth in the realms of the demigods and the gods too is
undesirable. This is because, although one experiences a certain
degree of happiness and power, existence amongst the demigods and
gods is impermanent. Besides, because of the distractions and
pleasures in these realms, beings there never think of looking for
a way out of the cycle of birth and death. This is why it is said
that of the six realms, the most fortunate, opportune and favored
is the human realm. This is why I have left the human realm to the
last.