A series of conflicts (1096-1204) in which European conscripts called by the Catholic Church attempted to conquer the Holy Land. The First Crusade (1096-1099) was actually an answer to the Byzantine Empire's (the surviving eastern half of the Roman Empire, and the largest Christian country at the time) call for assistance fighting the Muslims. Pope Urban II asked the Catholics to go on an "armed pilgrimage" to the Holy Land and establish a kingdom there. This call wasn't answered by any monarchs, and very few nobles or knights. For the most part, the "crusaders" were an angry mob of commoners such as peasants, merchants, and prostitutes. Somehow this mob managed to establish a kingdom in Jerusalem, which later fell to the Muslims. The Second Crusade (1147-1149) was a bit better organized, but was met with heavy Muslim resistance. The Third Crusade (1189-1192) was answered by King Richard I of England, King Philip II of France, and Frederick I Barbarossa of the Holy Roman Empire (Germany). Frederick drowned in a river on the way to the Holy Land, and Philip and Richard began fighting among themselves. The Fourth Crusade (1202-1204) was the last major one. These crusaders weren't even after the Holy Land this time; they sought to defeat the Byzantine Empire, and thus were fighting other Christians.
The fact that the Crusades even happened is sadly ironic, as Jesus' teachings were of love and peace.