In Mahayana Buddhism, a bodhisattva is a being is capable of liberating itself from the Wheel of Life (the need to reincarnate), but out of compassion has remained in the world, having taken the bodhisattva's vow not to pass off into Nirvana until all other beings have achieved liberation. A bodhisattva has committed himself or herself to help others reach that same state.
Various traditions within Buddhism believe in certain specific Bodhisattvas. Some bodhisattvas appear across traditions, but are given different names. For example, Tibetan Buddhism considers the Bodhisattva of Compassion to be incarnated as the Dalai Lama, while some other Buddhists believe in Kwan Yin.
Bodhisattvas are a very popular subject in Buddhist art.