Modes of Rebirth
A person may be reborn into any one of the following 31 realms or planes of existence, classified generally as the Formless Realms, the Form Realms, and the Sense-Desires Realms comprising the Happy States -- the human world is found -- and the Unhappy states:
The Formless Realms
Realms of Form
[Fourth Jhana Brahmas]
[Third Jhana Brahmas]
[Second Jhana Brahmas]
[First Jhana Brahmas]
According to Buddhism, until one has reached the first stage of spiritual development, i.e. Sotapanna or Stream-Winner, there is always the chance of being reborn into the Unhappy States. The Stream-Winner is the first of the Noble Disciples (Ariya Savaka) and has eliminated three of ten fetters to existence namely, notion of ego, sceptical doubt, attachment to (false) rites and rituals.
The Formless Realms and the Realms of Form are only reachable with the attainment of jhanas or meditative absorption. This is a state of intense mental purity brought about purely by the suppression of hindrances through concentration.
The Happy States consists of various planes where sensual pleasure may be enjoyed. One is reborn in any of these states dependent on oneís level of virtues. Although a great deal hellish suffering and beastly nature is found in the Human World, it is still known as a Happy State owing to the potential for sensual pleasure.
In the Unhappy States, pleasure of any sort is absent altogether. The only differences between the realms are levels of suffering. Ten evil actions (akusala kamma), killing, stealing, sexual misconduct, covetousness, ill-will, false or pernicious views, lying, slandering, harsh speech, frivolous talk, lead to these realms. Some people spend most lives holding false or pernicious views, and yet also spend much time doing good deeds. In Buddhism, these individuals are seen as being bound for the animal world, but with a very comfortable life, like a pedigree dog getting top quality food and care from its owner.