The Crusades were a Middle Ages "European Revolution" of sorts.
The Crusades, nine to be exact, took place from approximately 1096 to 1291 and were based on religious, political, social, demographic, and economic factors. The main goal of the Nine Crusades was to liberate the Holy Sepulcher from the Muslims and restore Christian possession.
Although nine crusades were given numbers, they were pretty much a continuous event throughout the 12th and 13th centuries. The major ones were primarily assigned by the Popes and kings of the ages against threatened Christian establishments or envied thronal areas, and just happened to last for quite awhile. The smaller, unnumbered crusades were carried out by, of course, less famous, or more so infamous, leaders and weren't as significant as the well known Nine.