Jodo Shinshu ("True School of the Pure Land") is a school of Pure Land Buddhism that was founded in 13th century Japan by Shinran (1173-1262). It is sometimes called Shin Buddhism or the Shin School. Today it is probably the most popular school of Buddhism in Japan as well as in Japanese ethnic communities around the world.
Pure Land Buddhism, sometimes called Amida Buddhism, originated in 5th century China. Very basically, Pure Land is a practice of devotion to Amitabha Buddha (in Japanese, Amida Butsu), the Buddha of Infinite Light. Faith in Amitabha leads to rebirth in the Pure Land, a place without the endless work and worry that interfere with the realization of enlightenment. It is a place, or else a transcendent state, from which Nirvana easily is reached.
The principal practice of Jodo Shinshu is the daily recitation of the Nembutsu, which is "Namu Amida Butsu," or "Veneration to Amitabha Buddha." Purity of devotion while chanting the Nembutsu is very important.
Pure Land Buddhism was brought to Japan in the 12th century by Honen Shonin (1133-1212), a Tendai monk. The school that Honen founded is called Jodo Shu.
Shinran, who also had been a Tendai monk, was one of Honen's students. He was a brilliant but problematic monk who married and fathered children, which was not yet permitted for monks in Japan. But Shinran saw his path as a teacher to laypeople, and he chose to live as a layperson. Shinran's primary disagreement with his teacher Honen was that while Honen encouraged multiple recitation of the Nembutsu, Shinran believed devotion was more important than repetition.