Short answers, pulled from the story.
The Crusades were a series of military campaigns launched by the papacy between 1095 and 1291 against Muslim rulers for the recovery and defence of the Holy Land. They were part of a wider crusading movement that promised spiritual rewards to participants.
Pope Urban II proclaimed the First Crusade at the Council of Clermont on the 27th of November 1095, calling Christians to reconquer Jerusalem from the Muslims. His appeal reportedly prompted the crowd to cry Deus vult, meaning God wills it.
The Fourth Crusade was diverted to Constantinople after the crusaders could not pay Venice for their fleet and accepted an offer from Alexios Angelos to restore him to the Byzantine throne. When payment failed, they stormed the city on the 12th of April 1204, leading to the Sack of Constantinople and the Latin Empire.
Saladin united Egypt and Muslim Syria, taking Damascus in 1174 and Aleppo in 1183. On the 4th of July 1187 he crushed the Jerusalemite army at the Battle of Hattin, and after a twelve-day siege Jerusalem surrendered to him on the 2nd of October 1187.
The Mamluks, having reunited Egypt and Muslim Syria by 1260, attacked the Crusader states and captured the Crusaders' last mainland strongholds in 1291. Plans for reconquest continued, but only the Alexandrian Crusade briefly revived crusading in the region in 1365.
The military orders began at the Council of Nablus in 1120, when Patriarch Warmund approved Hugues de Payens' confraternity of knights who vowed poverty, chastity, obedience, and protection of pilgrims. Housed in the former Al-Aqsa Mosque, called Solomon's Temple by the Franks, they became the Knights Templar.
The terminology evolved from words of movement like iter, expeditio, and passagium, reflecting the link between the Crusades and pilgrimage. Participants who sewed a cross onto their garments were called crucesignati, the earliest vernacular form crozada appeared in Spain in 1212, and the modern term crusade was established by 1706.