I recently finished reading the book "River of Fire, River of Water" by Taitesu Unno. This book provides an introduction to the Pure Land tradition of Shin Buddhism. In Japan, more people are followers of Shin Buddhism then of any other branch of Buddhism such as Zen or Shingon.
If you are interested in learning about the Shin tradition, then this is a pretty good book. The Buddhist terminology and philosophy can get a bit confusing and tedious at times, especially near the end of the book, but overall I learned a lot from it.
Pure Land Shin Buddhism is based on the belief in the Pure Land of Amida Buddha. Those who have faith in Amida Buddha will be born in Amida Buddha's Pure Land.
Pure Land Buddhism has been around along time and came to Japan from China. But a distinct school of Shin Buddhism was originally established in Japan by the monk Honen. Honen was a Tendai monk from Mt. Hiei, a center of Buddhist monastic study northeast of Kyoto. In 1175, he broke from this established center of monastic learning and proclaimed the establishment of an independent Jodo or Pure Land school.
Pure Land practice had long been a part of the established schools including those of Mt. Hiei. But Honen made the contemplation of Amida and the Pure Land a separate and distinct path that could be followed by all people including the common people and not just for those who followed the monastic path.
One of Honen's followers, Shinran (1173-1263) continued propagating the Pure Land belief after Honen's death. It was Shinran's decedents and followers that created the dominant Jodo Shinshu school of Buddhism that is the largest school in Japan today.
In real simple terms, Honen and his Jodo Shu school believed in the Nembutsu, the invocation of Amida Buddha's name, Namu Amida Butsu, as the way for those to travel to Amida's Pure Land. Shinran's school of Jodo Shinshu emphasized strong faith in Amida and that it was not even necessary to chant Amida's name but the mere the thought of the Nembutsu with strong faith was sufficient.