Foxes and human beings lived close together in ancient Japan; this companionship gave rise to legends about the creatures. Kitsune have become closely associated with Inari, a Shinto kami or spirit, and serve as its messengers. This role has reinforced the fox's supernatural significance. The more tails a kitsune has—they may have as many as nine—the older, wiser, and more powerful it is. Because of their potential power and influence, some people make offerings to them as to a deity.
It is widely agreed that many fox myths in Japan can be traced to China, Korea, or India. Chinese folk tales tell of fox spirits (called Huli-jing) that may have up to nine tails, or kumiho as they are known in Korea. Many of the earliest surviving stories are recorded in the Konjaku Monogatari, an 11th-century collection of Chinese, Indian, and Japanese narratives.
Kitsune are believed to possess superior intelligence, long life, and magical powers. They are a type of yōkai, or spiritual entity,and the word kitsune is often translated as fox spirit. However, this does not mean that kitsune are ghosts, nor that theyare fundamentally different from regular foxes. Because the word spirit is used to reflect a state of knowledge or enlightenment, all long-lived foxes gain supernatural abilities
There are two common classifications of kitsune. The zenko (善狐?, literally good foxes) are benevolent, celestial foxes associated with the god Inari; they are sometimes simply called Inari foxes. On the other hand, the yako (野狐?, literally field foxes, also called nogitsune) tend to be mischievous or even malicious. Local traditions add further types. For example, a ninko is an invisible fox spirit that human beings can only perceive when it possesses them. Another tradition classifies kitsune into one of thirteen types defined by which supernatural abilities the kitsune possesses.
Physically, kitsune are noted for having as many as nine tails. Generally, a greater number of tails indicates an older and more powerful fox; in fact, some folktales say that a fox will only grow additional tails after it has lived 100 years. One, five, seven, and nine tails are the most common numbers in folk stories. When a kitsune gains its ninth tail, its fur becomes white or gold. These kyūbi no kitsune (九尾の狐?, nine-tailed foxes) gain the abilities to see and hear anything happening anywhere in the world. Other tales attribute them infinite wisdom (omniscience). ( From: http://en.wikipedia.org/wiki/Kitsune )