Kami are the gods and spirits of Japan. The ancient Japanese followed an animistic belief system, known today as Shinto, in which all of nature - every animal, plant, or spring, planet or tiniest pebble, was possesed of an immortal spirit, or kami. These kami were to be venerated, respected, placated, or thwarted, as the situation demanded. Humans themselves became kami in death, and thus the Japanese worshipped their ancestors.
The greatest of the kami would properly be called "gods," possessed of mighty powers to shape and reshape the world. The mightiest of all was the kami of the Sun, Amaterasu, usually thought of as female (although strictly speaking the kami had no gender and could manifest in either male or female form). The mythic first Emperor, Jimmu, was the great-great-grandson of Amaterasu, and thus the Emperors of Japan were held to be living gods-on-earth. According to legend, it was Amaterasu herself, at the behest of the Emperor, who summoned the mighty Kamikaze storm that saved Japan from the Mongol Invasions.
Other kami of note include Amaterasu's miscreant half-brother, Susanoo, the kami of storms, Raiden, the kami of lightning, and the twin brother and sister kami Izanagi and Izanami, the first of the kami, who descended from heaven on a rainbow and created the universe. Naturally, Japan was created first; when Izanagi dipped his spear in the premordial ocean, the drips that fell from its tip formed the many islands of Japan.
According to Shinto tradition, there are eight million kami. However, the words "eight" and "million" both were used in olden times to denote "many," so this number might be better translated as "many, many." Many of the best known stories about the great kami are recounted in the Kojiki, the earliest known example of Japanese writing.