As described in The Oxford Encyclopedia of World History
A series of expeditions (11th -14th century) to secure Christian rule over the Muslim-controlled holy places of Palestine. (The term is by extension used to describe any religious war or even moral or political movement.) The wealthy powerful orders of Knights Hospitalliers and Knights Templar were created by the Crusades.
The First Crusade was called by Pope Urban II, and was provoked by the rise to power of the Seljuk Turks, which interfered with traditional Pilgrimage tp Palestine. The Pope promised spiritual benefits to warriors willing to fight under Christian banners. The Crusaders captured Jerusalem in 1099 and massacred its inhabitants, establishing a kingdom there under Godfrey of Bouillon.
The Second Crusade (1147-49) succeeded only in souring relations between the Crusader kingdoms, the Byzantines, and friendly Muslim rulers.
The Third Crusade (1189-92), prompted by Saladin’s capture of Jerusalem , recaptured Acre but achieved little more.
The Fourth Crusade (1202-04) was diverted by Venetian interests to Constantinople, which was sacked, making the gulf between Easter and Western churches unbridgeable, though some Crusaders benefited from the division of Byzantine territories known as the Latin empire of the East (1204-61). This briefly replaced the Greek empire at Constantinople until Michael VIII retook the city. Later expeditions concentrated on North Africa, but to little purpose. The fall of Acre in 1291 ended the Crusader presence in the Levant.
All, except the peaceful Sixth Crusade (1228-29), were marred by greed and brutality : Jews and Christians in Europe were slaughtered by rabble armies on their way to the Holy Land. The papacy was incapable of controlling the immense forces at its disposal. However, the Crusades attracted such leaders as Richard I and Louis IX, greatly affected European Chivalry, and for centuries, its literature. While deepening the hostility between Christianity and Islam, they also stimulated economic and cultural contacts of lasting benefit to European civilization.