According to Buddhist cosmology (based on the Abhidharma), samsara consists of thirty-one realms of existence. This means that there are thirty-one basic classes of beings that comprise the round of rebirth. Any being may be born at any one of these levels.

If one accepts rebirth as a working hypothesis, then every being has during the course of their wandering through samsara at some time or another been born in every one of these conditions – apart from the five realms known as the “Pure Abodes”. Beings born in these realms have reached a condition in which they invariably attain nirvana, and so escape the round of rebirth.

The thirty-one realms, from bottom to top, reflect a basic movement from gross to subtle.

A key question is: What determines in which realm a being is born? The basic answer is karma. It is a being’s intentional ‘actions’ of body, speech, and mind – that is, whatever is done, said, or even just thought with definite intention or volition.

Generally speaking, rebirth in the lower realms is considered to be the result of relatively unwholesome or bad karma, while rebirth in the higher realms is the result of relatively wholesome or good karma.

It should be noted that this hierarchy is not a simple ladder whereby one climbs or passes out at the top into nirvana. In fact, nirvana may be obtained from any of the realms, from the human to the highest of the Pure Abodes and the four formless realms – but not from the four lowest realms.

In reality, rather than attaining nirvana, beings generally rise and fall, and fall and rise, through the various realms – now experiencing unhappiness, now experiencing happiness. This, unfortunately, is the nature of samsara – wandering from life to life with no particular direction or purpose.

Source: Gethin, R. (1998) The foundations of Buddhism. Oxford: Oxford University Press. (Pages 115-119.)