Short answers, pulled from the story.
Kami is a fluid term that describes the wind, a mountain, a river, a specific ancestor, or a quality like fertility. Ancient scholars like Motoori Norinaga defined it as any being possessing an eminent quality that is awe-inspiring. This concept blurs the line between the divine and the natural world.
The Engishiki document was created in the year 927 CE and listed 3,131 official kami. This document illustrates the historical count of spirits before the number exploded into millions of individual spirits enshrined in places like Tokyo's Yasukuni Shrine.
Amaterasu is the sun goddess who sent her grandson Ninigi-no-Mikoto to earth to rule in the creation myths recorded in the Kojiki and the Nihon Shoki. She is the ancestor of the first emperor and her lineage established the imperial family as direct descendants of the kami.
The phrase yao-yorozu no kami means eight million kami and refers to the concept of uncountably many spirits that exist in the world. The number eight is used symbolically to represent all directions, suggesting that kami are present everywhere from the highest peaks to the deepest oceans.
Misogi is a spiritual cleansing of the heart and mind required to gain magokoro or a pure sincere heart. The ritual begins with the washing of hands and the rinsing of the mouth followed by the spitting of water to symbolize the expulsion of impurities.
Sugawara no Michizane was a scholar and statesman who was exiled and died on the 2nd of May 903 CE. He was deified as Tenjin the god of learning and poetry after a series of terrifying omens and disasters were attributed to his wrath.