The Bodhisattva ideal in practice (and its relation to the Arhat ideal)
The essential aspiration of a bodhisattva is bodhicitta, the vow or intention to achieve enlightenment out of compassion for all those suffering in samsara, which contrary to popular perception does not necessarily imply being reborn until all beings in samsara achieve enlightenment, since the Buddha, who was clearly a bodhisattva, is not (and cannot be) reborn, but he did live many many lives before achieving Buddhahood. The difference between a stream entrant, who seeks arhantship, and a bodhisattva is that a stream entrant seeks enlightenment for himself and seeks to achieve emancipation as soon as possible, thus freeing himself from the bondage of samsara quickly. The Buddha taught that it is possible to do this in not less than seven days and not more than seven rebirths, but the Pali Canon records instances of attainment of arhantship in as short a time as five days. Arhantship has the net effect of reducing dharma in samsara, since the arhants are not reborn, reducing the overall quantum of dharma, whereas bodhisattvas are reborn many many times - perhaps infinite numbers of times potentially but not necessarily - thus increasing the quantum of dharma in samsara. Thus, the arhant ideal leads to degeneration (entropy) whereas the bodhisattva ideal leads to evolution (negentropy).