What is the main idea behind the shinto religion?

What is the main idea behind the shinto religion?

I can best describe it as harmony with nature. Understanding the place of humanity, and its relation to the natural world. It's hard to look at Shinto, anymore, without looking at the Buddhism that has so decorated and shaped it. The distinguishing factor, however, is the focus on the here and now--and not an afterlife.
Also, in Shinto, there are Gods for everything. There are guardian Gods that people may choose, like Bishamonten, Marici, and Amaterasu, and there are Gods of trees, and greater Gods of forests. There's a small hierarchy, in that--a tree god would be seen as less powerful than a forest God, or a River God. And it's not like, One God of all rivers, or forests either--it's each river has its own God. Each Province has its own God. It's a beautiful religion, and the best way I can explain it is to point you in the direction of two Miyazaki animations--Princess Mononoke, and Spirited Away. Those two are very good representations of the Shinto faith.

Its a naturalistic shamanistic religion, everything is 'God' in its way, as in the world is god and everything in it is god.

Shinto is considered Japans native religion. A system of simplicity and beauty, the main ideas behind Shinto are rooted not in the after-life, like many other popular belief systems, but rather in finding harmony with your current surroundings in the present. There is no strict dogma or prayer, and no hierarchy of Gods to worship. Rather, Shinto is a collection of rituals and methods to strengthen relationship of living humans and Jami, also known as spirits. Some Jami can be specific to local customs, others, are larger, shared natural objects such as Amaterasu, the Sun goddess, or Mount Fuji. But the general understanding is that everything contains a Jami.

http://www.colourlovers.com/blog/2008/06/19/colors-of-religion-shinto/

As to the origins, that is more difficult to understand, for the Chinese tried their best to wipe it out, and there was a revival of Shinto during the Edo period, which was about the time Catholicism and Christianity were introduced to Japan. For the most part it appears to be a cultural practice more than a religious one, but with the spiritual world having a large influence on the daily lives of the people.