In Mahayana Buddhism, bodhisattvas are those who are on the path to Enlightenment, but have not yet attained it and become buddhas. Any living person who has embarked on the Bodhisattva path can thus be considered a bodhisattva.
The celestial bodhisattvas (those who dwell in the heavens) are those who are advanced enough to attain enlightenment at any time, but who have renounced final Enlightenment in order to help other beings.
Bodhisattvas are thus characterized by compassion and can be relied on to help those on the Buddhist path in their various ways. One of the most important bodhisattvas is the savior-goddess Tara, who the principal deity of Tibet.
The following chart summarizes the identities and characteristics of the most important bodhisattvas in Buddhism.