A bodhisattva is a being bound for awakening who has generated bodhicitta, the mind set on becoming a fully awakened Buddha for the benefit of all sentient beings. The generation of that intention, formalised in a vow taken before a teacher or before the Buddhas, is the entry point of the Mahayana path and the thing that distinguishes it.
The career is described as immensely long, running through ten stages, bhumis, over a span of aeons, and it is structured by the six perfections: generosity, discipline, patience, energy, meditation and wisdom, with four more added in some schemes. Wisdom, the direct realisation of emptiness, is the perfection that makes the others perfections rather than merely virtues; the tradition says repeatedly that generosity without insight into the emptiness of giver, gift and recipient is meritorious but not liberating.
The doctrine of merit transfer follows from the vow and does a great deal of work in Mahayana practice. Because the bodhisattva acts for others, the merit generated by practice can be dedicated to them, and the accumulated merit of the great bodhisattvas is available to those who appeal to them. This is the mechanism underlying much East Asian devotion, and it is a genuine departure from the earlier texts, where the fruits of action attach to the one who acts.
Certain bodhisattvas became objects of veneration in their own right: Avalokiteshvara, the embodiment of compassion, who transformed into the female figure Guanyin in China and Kannon in Japan; Manjushri, of wisdom, holding a sword that cuts through ignorance; Kshitigarbha, who vows not to rest while any being remains in the hells; and Maitreya, the Buddha of the next age.