There were several crusades spread over several hundred years, and the reasons they were initiated varied, although almost all were the result of Papal exhortations; in fact the First Crusade began because of Papal exhortation.
The first Crusade was initiated by the Preaching of Pope Urban II at the Council of Cluny, France. Urban had received a message from the Byzantine Emperor about the advance of Islamic forces and asked for help from the West. Urban saw this as not only the chance to aid Christians in Byzantium but also as a chance to reunite Christianity (East and West) under his leadership. Additionally, he saw this as an avenue to vent the belligerent tendencies of Western European nobility who were trained to do nothing but fight.
In his sermon, Urban not only describe horrors presumably committed by Islamic forces (for instance, using Christians tied to poles as target practice) but he also promised complete remission for any sin committed while on Crusade. Additionally, the Crusade was styled as a pilgrimage which in itself was an act of absolution. Although this was the promises made by Urban, there is unquestionably strong evidence that many knights went on Crusade solely for adventure and the hope of plunder and reward.