Shinto translates as "The Way of the Gods" ('Shin' means Gods/Spirits, 'To' means Way), and there are many gods (or spirits) in Shinto. Shinto is based on belief in, and worship of, kami -- which are loosely related to spirits. The idea is that everything animate has Kami (some good, and some bad), but you care about some Kami more than others (ancestors, people of great merit, some of the Kami that created the earth, and so on). There are so many spirits in the Shinto faith, that a Japanese expression is "yaoyorozu-no-kami" or "countless spirits" (eight million spirits).
Harmony, in any form, is one of the major principles of Shinto; whether the harmony be of the gods, nature, or community. Assisting this harmony is good, causing turmoil or chaos is bad. This has allowed Shintoism to integrate or allow impressions of other religions on to it, particularly Buddhism and Taoism. There is also a strong sense of "Duty" and "Responsibility"; to ones superiors, peers and inferiors (resembling the Confucianistic ethic). There are also some Shintoists that have interwoven Christianity into their religion. With only slight changes in the personification of deities, things mesh pretty well.
There are many rituals and festivals associated with Shintoism and through-out the Japanese culture. Shintoists believe that after the soul departs from the body, memorial rights free that soul of it burdens, bringing it peace. Many of these spirits and gods may take residence in objects of worship (Shintai), be it a tree, a sword or a mountain. There are many shrines, often protected by an individual deity or spirit.
There is no "founder" of Shinto and there is no supreme book or reference, this makes shinto very adaptable. The ideals of harmony being so infused into the religion has also helped Shinto adapt. So Shinto has become a dynamic religious system, and has changed greatly with the times.