The Crusades were a series of military campaigns that occurred from the eleventh through the thirteenth centuries. They were the result of the Turks' military defeats of the Byzantine Empire, causing the Holy Land and most significantly Jerusalem to fall under the full control of Muslims. Although the avowed purpose of the Crusades was religious, other motivations included both geopolitcal power and the opportunity to loot and pillage (including sacking Constantinople which the Crusaders in theory were rescuing).
Most of the Crusades were organized under the aegis of the Pope, leader of the Roman Catholic Church. In contrast, the People's Crusade, sometimes also called the Paupers' Crusade or Peasants' Crusade, occurring in 1096 was was slightly earlier than the actual First Crusade organized by Pope Urban II, and was more of a spontaneous movement.
Led by Peter the Hermit, the People's Crusade set out for Jerusalem in April 1096, slaughtering members of French and German Jewish communities in its path, despite the objections of local Roman Catholic clergy and the Pope himself.
After a rather disorganized march, involving pillaging, riots, and internal dissension, the People's Crusade arrive in Constantinople and then attacked the Turks at Nicaea in October 1096, a battle they lost, marking the end of the People's Crusade.