There were seven Crusades in all, dating from 1095 to 1291, with the First Crusade being the most successful from a military point of view.
The First Crusade was an attempt to re-capture Jerusalem. Christians had paid pilgrimage to the Holy Land since the time of Constantine. After Jerusalem had been captured by the Saljuk Turks in 1076, any Christian who wanted to pay a pilgrimage to the city faced a very hard time with the Moslem soldiers making life very difficult for the Christians. Trying to get to Jerusalem was filled with danger.
Emperor Alexus I of Constantinople, modern day Turkey, feared that his country might fall to the Moslems as they were very close to another territory that had already been captured. Alexus called on Pope Urban II for help. On November 27th, 1095, Urban spoke to a crowd at the Council of Claremont in France. In this speech, considered the most influential speech of the Middle Ages, he called for a war against the Moslems so that Jerusalem could be regained. He promised that any volunteer who participated would be forgiven their sins and asked for each warrior to wear a cloth cross on his tunic.
Those who volunteered to fight the Moslems cut out red crosses and sewed them on their tunics. The French word "croix" means cross and the word changed to "croisades" or crusades. The Holy War was begun.