1. Buddhism: The Human Condition
Samsara
Buddhism: The Human Condition

2. Samsara Means the cycle of life, death, and rebirth.
Unenlightened beings are trapped in the cycle of life, death and rebirth because of greed, hatred & ignorance (the 3 root poisons).
We can be born into one of six realms:
Human Realm
Realm of the Hungry Ghosts
Animal Realm
Deva (gods) Realm
Ashura (demi-gods) Realm
This is illustrated in the wheel of samsara.

3. Wheel of Samsara

4. Human Realm
The Human realm is the best realm to be born into, as it is only from this realm is it possible to attain enlightenment as a Buddha. (A Buddha being a person who has attained enlightenment without the help of the teaching of others).
Theravada
An arhat (who has the help of teachers) can attain enlightenment from the realms of the devas.
Mahayana
A bodhisattva can appear in many different types of lives, for instance as a human an animal or as a deva.

5. Realm of the Hungry Ghosts
The hungry ghosts are just that, constantly hungry and thirsty but never able to satisfy their hunger or thirst.

6. Animal Realm
It takes many lives in the animal realm to build up enough merit to be reborn as a human.

7. Deva (gods) Realm
The Deva realm is also sometimes called the realm of the gods, because its inhabitants are so powerful within their own realm, that compared to humans, they resemble the gods of Greek or Roman mythology. They are not gods in the way we normally think of them.
Mahayana
The disadvantage of this realm is that things are so very comfortable there, that these beings completely neglect to work towards enlightenment. Instead they gradually use up the good karma they had previously accumulated, and so they subsequently fall to a lower rebirth.
Theravada
They only have 5 realms because they count the Deva & Ashura Realms as one Realm.
According to Theravada tradition it is possible to attain enlightenment from this realm.

8. Ashura (demi-gods) Realm
The Asura realm is the realm of the demigods (half gods).
They may be here because in human form they had good intentions but committed bad actions such as harming others.
The Asuras are said to experience a much more pleasurable life than humans, but they are plagued by envy for the devas, whom they can see just as animals can perceive humans.
Theravada
They only have 5 realms because they count the Deva & Ashura Realms as one Realm.
According to Theravada tradition it is possible to attain enlightenment from this realm.

9. Hell Realm
The Hell Realm is a rebirth based on strong states of hatred cultivated in a previous life or lives.

10. Samsara
In Buddhism, there is no reincarnation as there is no soul to be reborn.
Rebirth in Buddhism is more like a transfer of energy. Think about one pool ball hitting another, the first one stops dead and the ball which has been hit travels on. Another way to imagine it is lighting a new candle with the flame of an old one.
Sometimes this energy can carry a memory with it (see Tibetan Buddhist idea of Tulkus / Dalai Lama etc).