@mister master--Simply put, Kages may be people that current Kages gave their title to, or it may be earned through bloodline, i.e., how Gaara became Kazekage because of his father being the previous.
Here's an excerpt from wikipedia on Kages:
Originally Posted by Wikipedia stub
Those who have achieved the title of Kage (影, Kage? literally "Shadow") are the leaders of the five most powerful Hidden Villages. Kage are the most powerful ninja of the village. There may be exceptions -- for example, a ninja may be the most powerful at the time of being installed as Kage, but he will age and a more youthful ninja may become more powerful. Kage are an elite group as there can only be one per village at any given time.
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 Hokage and the Third Hokage, where the latter had given up the title to the former, only to come out of retirement when the former died.
Their elite status is also attributed to the tendency for most Kage to have family members who were previous Kage, or be personally taught by a former Kage or student of a former Kage. Examples of this occurrence include Gaara, who succeeded his father as Kazekage; the Fourth Hokage, who was taught by the Third Hokage's student Jiraiya; Tsunade, granddaughter of the First Hokage, grandniece of the Second Hokage, and former student of the Third Hokage; and the Third Hokage who was taught by both the First Hokage and Second Hokage.
For the full article, click here: http://en.wikipedia.org/wiki/Hokage#Kage