Urban II preached the crusade at the Council of Clermont in November 1095, and then continued preaching it through southern and central France for the following few months. He also wrote letters to clergy in other parts of Europe. The audience in Clermont and elsewhere had been mainly clerical clergy, bishops and preachers, who went on to preach the crusade throughout Europe. The ultimate target audience for the call to a crusade was the knightly class of western Europe, which certainly came to be aware of the it. The language used in the call for a crusade struck a chord in those knights, who saw Christ as a king deprived by the Muslims of lands that were rightfully his, and recognized the need for revenge, similar to a call for a vendetta that they would have followed if the land had been taken from their own lord. The call for a crusade in those terms really fit with the culture of fealty of the upper class of the late eleventh century. This helped the preaching of the crusade spread like wildfire. There are certainly other elements, a subculture of millenarism in the passionate preachers of the era being one of them. The result was that in the two years following Clermont, people from all parts of western Europe, from France, but also from England, Flanders, Germany, northern and southern Italy (notably not Spain, as Urban requested that they fight the Muslims at home instead of going to Jerusalem, and granted them the same benefits as those going on the crusade), took the cross and mortgaged or sold their family lands to raise money for an armed pilgrimage to the Holy Land.

Riley-Smith - The First Crusade and the Idea of Crusading is an excellent book on the topic.