A Kage (影, Kage) is the leader of one of the five most powerful Hidden Villages and generally are also the most powerful ninja in their respective villages. Though there may be exceptions such as a ninja may be the most powerful at the time of being installed as Kage, but has since aged and a more youthful ninja may have become more powerful.
A Kage may retire and give the title to someone else; the title, once granted, is held permanently. This creates a case where there are two Kage at the same time, with only one of them actually active, as was with the Fourth and Third Hokage, where the latter had given up the title to the former, only to come out of retirement when the former died.
The Kage stand equally with the rulers of their countries, oversee the activities of their villages, and are the leaders of ninja society. They typically are the ones who send the ninja teams out on their missions and make the decissions regarding the saftey of the village.
The Star Village is one exception of a village not among the five most powerful which has had a kage for special reasons.
The five Kage have different titles depending on their village. Those titles are: