Medieval Europe saw Jerusalem as the center of the world, as in this medieval map
(from pages 90-91 of Chronicles of the Crusades).
To see this map in more detail click on the map--be forewarned that the image is large and may load slowly. Jerusalem was believed to be the setting for the last days, as described in the book of Revelations. Furthermore, during the eleventh century (when the crusades began), many Christians believed that Domesday would occur about a thousand years after the New Testament was finished and judgment would occur in "Jerusalem, in Christian hands" (Chronicles 60).
The crusades were not just a military effort to control territory, but were also a pilgrimage. Members of the First Crusade at Jerusalem, before sacking the city, performed a penitential procession, visiting holy places outside of the city proper. Going on crusade could also be considered a penance and way of earning forgiveness. Papal indulgences were promised (and probably delivered) to those who went on crusade (and made it to Jerusalem).
Jerusalem is also a holy city for the Muslims, who classify it as the third holiest city, after Mecca and Medina. They believed that Judgment Day would occur close to Jerusalem, but outside, near the eastern wall (Riley-Smith  223)
A listing of the kings of the Frankish kingdom of Jerusalem can be found here.
Another site with several maps of the area from different time periods (including during the Great Crusades) is http://www.israel.org/mfa/maps.html.