There are six realms in which one may be reborn after death. They are the realms of gods, the demigods, human beings, animals, hungry ghosts and the hells. These are just general categories and within each, there exist many sub-categories. The six realms of existence include three relatively happy states, and three relatively miserable states. The realms of the gods, the demigods and human beings are considered to contain more happiness and less suffering. The realms of animals, hungry ghosts and the hells are considered to be relatively miserable because living beings there suffer more from fear, hunger, thirst, heat, cold and pain.
In general, wholesome actions such as good conduct, charity and mental development, are the causes of rebirth in the happy realms of gods, demigods and human beings. On the other hand, unwholesome actions such as immoral conduct, miserliness and cruelty cause rebirth in the unhappy realms of animals, hungry ghosts and the hells.