Jōdō Shinshu ( 浄土真宗 ; "True Pure Land School"), also known as Shin Buddhism, is a Japanese School of Buddhism which grew out of the Pure Land schools (specifically the Jōdo school of Hōnen) that emphasizes entrusting in Amida in regards to attaining enlightenment as opposed to engaging in practices of self-effort (such as meditation) as found in most other Buddhist schools of the time. The school's thought was developed by Shinran in 1224, which was during the Kamakura period in Japan, and was further refined by Rennyo.
Important texts in the Pure Land stream include the Amitabha Sutra, The Smaller Pure Land Sutra, and the Larger Pure Land Sutra. Within Shinran's school, his writing, Ken Jōdō Shinjitsu Kyogyoshō Monrui (also Kyogoshinshō, "The True Teaching, Practice, and Realization of the Pure Land Way") and a latter work of uncertain authorship, the Tannisho ("Lamentation of Deviations from the Way"), comprise two major texts.
The Shin school is rooted in the vows of Amida Buddha, whose name in Japanese represents a fusion of the Sanskrit Amitabha ("Limitless Light") and Amitayus ("Limitless Life"). Prior to becoming a buddha, Amida was known as the bodhisattva Dharmakara. The key vow that Dharmakara Bodhisattva made, also known as the Primal Vow, is that he will not attain Buddhahood unless all those who call upon him with a sincere mind, joyful trust, and a firm desire for birth are born into his Pure Land after death. After an incomprehensible amount of time, Dharmakara's pure practice developed him into full Buddhahood, bestowing upon him the power to make manifest his vows.
Shinran's thought was also influenced by an understanding of mappo (末法), or the decline of the Dharma (the Buddhist teachings). Shinran saw the age he was living in as being in a degenerate age where beings cannot hope to be able to extricate themselves from the cycle of birth and death through their own power, or jiriki (自力). For Shinran, all conscious efforts towards achieving enlightenment and realizing the Bodhisattva ideal were contrived and rooted in selfish ignorance; inauthentic in nature, for humans of this age and beyond are so deeply rooted in karmic evil as to be incapable not only of attainment but also of the truly altruistic compassion that is requisite in becoming a Bodhisattva.
Thus, Shinran advocates tariki (他力), or reliance on Other Power -- the power of Amida Buddha's limitless and infinite compassion made manifest in the Primal Vow -- in order to attain liberation. Shin Buddhism can be understood as a "practiceless practice," for there are no specific acts to be performed such as in the "Path of Sages" (the other Buddhist schools of the time that advocated self power).
The nembutsu (念仏): Namu Amida Butsu (南無阿弥陀仏) ("Hail to Amida Buddha") chanting practice common within the other Pure Land schools is seen in a new light. The nembutsu becomes understood as an act that expresses gratitude to Amida Buddha -- furthermore, it is evoked in the practitioner through the power of Amida's unobstructed compassion. In Shin Buddhism, the nembutsu is not considered a practice, nor does it generate karmic merit.
The goal of the Shin path, at least the practicer's present life, is the attainment of shinjin (信心 True Entrusting) in the Other Power of Amida. To achieve shinjin is to unite one's mind with Amida through the total renunciation of self effort in attaining enlightenment; to take refuge entirely in Other Power. Shinjin arises from jinen (自然 naturalness, spontaneous working of the Vow) and cannot be achieved soley through conscious effort. Shinjin develops over time through "deep hearing" of Amida's call of the nembutsu. Jinen also describes the way of naturalness whereby Amida's infinite light illumines and transforms the deeply rooted karmic evil of countless rebirths into good karma. It is of note that such evil karma is not destroyed but rather transformed: Shin stays within the Mahayana tradition's understanding of sunyata, or non-duality / emptiness, and understands that samsara and Nirvana are not seperate. Once the practicer's mind is united with Amida and Buddha Nature gifted to the practicer through shinjin, the practicer attains the state of non-retrogression, whereupon after his death he will achieve instantaneous and effortless enlightenment. He will then return to the world as a Bodhisattva, that he may work towards the salvation of all beings.
Despite possessing some radical deviations from his master Honen's ideas, Shinran always saw himself as espousing his teacher's views and he held his master in high regard. Shinran spent his early life traveling throughout Japan (somewhat against his will initially, as he and his master were both punished for threatening the Buddhist establishment with exile to various areas) and preaching to the peasant class, who found Shin Buddhism to be attractive due to its non-exclusivity and openness to all classes and peoples. Later in life he settled down and devoted his efforts towards writing.
Jōdō Shinshu itself has many sub-sects. The largest branch is the Hongwanji-ha sect. This sub-sect is further divided into two types, Hongwanji-ha Hongwanji, shortly called Honpa Hongwanji (or more commonly, Nishi-Hongwanji) and Daiha Hongwanji, or Otani Hongwanji, more commonly referred to as Higashi-Hongwanji. None of these can be broken down further, and Nishi is the larger of the two. In the United States the Nishi-Hongwanji operates as the Buddhist Churches of America.