According to Buddhist beliefs, after one's death, one must be born again to receive punishment for the evil deeds one has done in the previous lives or to enjoy rewards for one's good deeds. But in the case of the Buddha, each time he was born, he had a chance to perform good deeds and thus he piled up the graces and virtues from all his previous lives until he finally attained Enlightenment and became the Lord Buddha.
Jataka stories are a helpful means by which Buddhist principles are explained and pointed out in story form. These stories are interesting and entertaining and so are a very efficient means of teaching morals to adults as well as children, comparable to India's Hitopadesa or Europe's Aesop's Fables. Jataka stories, however, are longer, more complicated and more amusing. In Thailand there are two main groups of jataka stories: the ones which can be found in or traced back to the Tripitaka and those which were made up and written down in the Pali language in most Chaing Mai only 300-400 years ago.
The jataka stories in the Tripitaka are divided into groups (nibat) according to the length and the number of katha(a verse or stanza in Pali) :
1) Ekanibat
Each jataka story in this group is written in only one katha.
2) Tukanibat
Each story is written in 2 katha.
3) Tikanibat
Each story is written in 3 katha.
The jataka stories are grouped in this way all along according to the increasing number of katha. Eventually we come to the final group called Mahanibat. The jataka stories in this group contain more than 80 katha each. There are ten such jataka stories altogether and the Thai people usually call them, collectively, Tosa-jataka(meaning"Ten Jataka stories)