Crusades were military campaigns sanctioned by the Catholic Church during the Middle Ages, to expand the realm of Christianity or to put down heresies within it. The Crusades most commonly refers to the nine crusades to the Holy Land from 1095 to 1272.
While the headline summation is "Roman Catholic Europeans going to the Holy Land for religious and economic reasons to conquer" and it is relatively well known that a speech by Pope Urban II in 1095 launched the first crusade, the actual history is much more complicated. Pilgrimage to the sites of the life of Jesus of Nazareth had been a Christian tradition at least since the trip of Emperor Constantine's Christian mother to Jerusalem in the 4th century where she claims to have found the "True Cross of Christ" (splinters of which would later add up to a veritable forest of crosses). However with the Byzantine Empire fighting an on again off again war against Muslim adversaries to the East, the rulers in Constantinople started toying with the idea of asking the Christians to their west to aid them in defeating the "heathens" to their east. Reports of pilgrims being harassed and even a desecration of Holy Sites further fueled the desire by Europeans to take revenge for real or perceived wrongs.
Other crusades include Northern Crusades to Balticum and Finland, the Abligensian Crusade against the Cathars of what is now southern France, the Reconquista taking the Iberian Peninsula back from the Moors, and several crusades against the Hussites.
The Crusaders created numerous states of varying longevity. The nominal claim to the Kingdom of Jerusalem was maintained in the long form titles of several monarchs into the early modern period. Indeed, some crusades served mainly to help out a Crusader state in need. In "Outremer" as the Crusader territories came to be collectively known there were often tensions between Catholic new arrivals and Jewish and Muslim inhabitants but also sometimes the non-Catholic Christians who did not mind their erstwhile Muslim rulers but did not like being bullied around for "doing Christianity wrong".