Among Buddhists, the main goal of life was to become enlightened so that you could stop being reincarnated when you died, and instead enter nirvana (like Heaven) and be one with God. Boddhisatvas were people who had achieved enlightenment, and could enter nirvana, but 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. Some other famous bodhisatvas are Kuanyin and Jizo.
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