A jinja is a Japan
The term comes from "yashiro" which means a place for some type of building. In ancient times, rites to acknowledge and worship kami were performed outdoors.
A piece of land was chosen and roped off into a square and a tree was erected as an object on which the kami was invited to alight. This area, including the tree, was called "himorogi". When a rock was used instead of a tree, the place was called "iwasaka".
This ancient style of ritual can still be seen in Jichinsai, a rite performed before constructing a building.
When Buddhism was brought to Japan and patronized by the Soga clan, they put an image of the Buddha inside of a building, in the style of China and Korea. It is thought that Shinto was influenced by this, and started to enshrine kami in buildings during the 600s.
The jinja usually consists of several structures built from natural, unpainted wood and with thatched roofs. The buildings are torn down and reconstructed every twenty years.
At the centre of the jinja there is usually a metal mirror or some twisted bits of paper. This is not the kami or even a representation of the kami. It is just a way of referring to it.