Any European military expedition to free the Holy Land from Muslim occupation that was sanctioned by the Western Patriarchy (Roman Church, or Roman Catholics as they are known now). These expeditions occurred during between the 11th century and the 14th century.
The First Crusade was preached by Pope Urban II in 1095.
The Second Crusade was preached by Pope Eugene III in 1145.
The Third Crusade was preached by Pope Gregory VIII in 1187.
The Fourth Crusade was preached in 1198 by Pope Innocent III.
The Fifth Crusade was preached in 1216 by Pope Honorius III.
The Sixth Crusade was the product of Frederick II, Emporer of Germany (Holy Roman Emporer) commencing approximately 1225.
The Seventh, and final major Crusade was the work of King Louis IX of France, and departed Europe in 1248.
The Eighth Crusade finished in 1270, having being fought mainly by mercenary troops as there was little land to offer as enticement.
Other "Crusades" include Peter the Hermit's "People's Crusade" that actually preceded the First Crusade by virtue of setting off and "crusading" against non-Christians in Europe firstly.
The Crusade of 1101 was a dismal failure, and was little more than a second wave of the First Crusade that was poorly co-ordinated and resulted in three major European armies being defeated by Turks.
The crusades had multiple causes - Church reform, increased population in Western Europe, and the growth of the idea of knights as soldiers of the church. The success of the early crusades was due partially to division among the people occupying the Middle East.