A bodhisattva is the embodiment of bodhichitta -- the compassionate wish to realize enlightenment for all beings, not just oneself.
There are bodhisattvas who are transcendent beings of art and sacred texts, and there are bodhisattvas who are ordinary human beings. Bodhisattvas vow to remain in the world until all beings are enlightened and may enter Nirvana together.
For Mahayana Buddhists, a bodhisattva is the ideal of practice. In many schools of Mahayana, clergy and laypeople alike take the Bodhisattva Vows. There are many variations and translations of the vows; this is just one:
Beings are numberless; I vow to liberate them.
Delusions are inexhaustible; I vow to end them.
Dharma gates are boundless; I vow to enter them.
The Buddha Way is unobtainable; I vow to obtain it.
See also "What's a Bodhisattva?"