Previous Lives (Jataka Stories)
Once upon a time, many thousands of years ago, there was a Brahmin ascetic named Sumedha living in the Himalayan foothills. He was well advanced in his ascetic and meditation practices. It was the time of the previous Buddha Dipankara. When he heard that the Buddha was to visit the lowlands, he made his way down from his hermitage in order to meet the Buddha.
He helped the people in making preparations for welcoming Buddha Dipankara. When he met the Buddha, the ascetic Sumedha was very impressed and thought, ’May I be able one day to attain Enlightenment like this Buddha, and help the people by teaching the Dhamma’. Buddha Dipankara read the mind of Ascetic Sumedha, and announced to those gathered there, ‘ This ascetic will one day become Enlightened as a Samma Sambuddha named Gotama Buddha’.
Ascetic Sumedha was delighted to hear these words, and determined there and then to work towards progress as a Buddhist pilgrim and become a Samma Sambuddha. Life after life he practised and perfected the qualities, paramis (paramitas), needed to progress on this path.
After Enlightenment, on different occasions, Gotama Buddha recounted the stories of his previous lives, to illustrate different aspects of his teaching. He often identified the persons with him in his current life with the characters in the story. These are the Jataka stories (or tales from the previous lives of the Buddha). The bodhisatta (bodhisattva) was born as a human being, animal, bird and fish, male and female, over many lives. The theme that runs through the Jataka stories is the persistent effort to perfect those qualities which led him to attain Enlightenment as Samma Sambuddha. There are about 550 Jataka stories.
Source: http://www.thebuddhistsociety.org