The Larger Pure Land Sutra tells of a monk named Dharmakara who, before a Buddha of a former age, made a series of vows and declared that he would not accept full awakening unless they were fulfilled. The eighteenth of them, the one on which the whole tradition rests, states that if beings who sincerely entrust themselves to him and call him even ten times are not born in his land, he will not become a Buddha. He did become a Buddha, Amitabha, of infinite light and infinite life; therefore, the tradition reasons, the vow is accomplished and the way is open.
The Pure Land, Sukhavati, is a realm established by his merit in which the conditions for practice are unobstructed: no distraction, no want, and the Buddha himself teaching. It is not a final heaven. It is a place from which awakening is assured, which is a different claim, and it explains why the tradition can be devotional in tone without ceasing to be Buddhist in structure.
The practice is the nembutsu, in Chinese nianfo, the recitation of the name, in Japanese namu amida butsu. The reasoning behind making it central is a doctrine of decline: in the degenerate age known as mappo, the argument runs, the disciplined monastic path is beyond the capacity of nearly everyone, and a way suited to the actual condition of ordinary people is needed.
Hence the tradition's organising distinction between self-power, jiriki, the effort of one's own practice, and other-power, tariki, reliance on Amitabha's vow. Pure Land teaching holds that the age calls for the second.