Jerusalem was a holy city that the Christians, Jews and Muslims wanted control over. The Christians believed that Jerusalem was holy because that was where Jesus was crucified and resurrected. Jewish people believed it was God's own city and site of Soloman's temple. The Muslims believed that it was 3rd holiest city and where Muhammad ascended to heaven. The Crusade's were a Christian group that fought for the religious right of Jerusalem.
Pope Urban ignited the Crusades when he said, "Expel that wicked race from our Christian lands... Christ commands it." Crusade actually has latin in it "crux" means cross. The Crusades ended up being a series of nine military expeditions. Why would people go? Well knights could finally use the techniques they learned, peasents had freedom from feaudal lands, and anyone else that wanted to go could get instant salvation if killed, plus the chances of adventure and possible wealth were out there.
The Crusades were a series of bloody wars for religious reasons. Pope Urban lead the people through this and everyone could get a reason to go. Would you?