There were eight or nine crusades, though only the first four were of any importance. The Crusades were Christian military expeditions mounted to wrest the Holy Land from the Muslims who had conquered it.
The First Crusade began in 1096, led by Peter the Hermit, a French priest. His followers were ill-trained and fared badly, but the knights who went to the Holy Land at the same time captured Jerusalem.
However, Jerusalem was lost and the Second Crusade began in 1147. It also failed. The Third Crusade was personally led by European rulers. Richard the Lion-Heart defeated Saladin and captured Acre, just north of Jerusalem. Historians consider the somewhat more successful Fourth Crusade as purely mercenary.
The other Crusades, including the notorious Children's Crusade, failed, and the Holy Land remained under Muslim control.