What is a boddhisatva?
Buddhists who were enlightened
Among Buddhists, the main goal of life was to become enlightened. Once you were enlightened, you could stop being reincarnated when you died. Instead, you could enter nirvana (like Heaven) and be one with God. Boddhisatvas were people who had achieved enlightenment, and could enter nirvana. But they chose to stay on Earth anyway so they could help other people towards enlightenment.
In his previous incarnations, before he was the Buddha, the Buddha was a boddhisatva. A lot of Jataka Tales are stories about the good deeds the Buddha did when he was a boddhisatva. But there are lots of other famous boddhisatvas. Many of them are from China, where Buddhism spread in the 400s AD. Some famous Chinese boddhisatvas are Guanyin and Jizo.
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More about Guanyin
More about Nirvana
And more about Buddhism