Pope Urban II was the Pope who had initiated the Crusades. The explanation was that the Seljuk Turks had captured over most of the Byzantine Empire, the Byzantine Emperor Alexius I begged Pope Urban II for Christian knights to help him battle the Turks. Pope Urban accepted and appealed for a crusade at the Clermont Council in 1095.
Peter the Hermit was a French monastic founder and preacher from the middle ages. His fiery intervention ignited the passions of the paupers (underprivileged masses) and persuaded them to travel with him to the east as part of the People's Crusade. Peter also gave an impassioned speech to the weary and defeated crusaders during the siege of Antioch, which ended in a glorious triumph over a greater Muslim army.
The Seljuks were a tribe of migratory Turkish warriors from Central Asia who settled themselves in the Middle East in the 11th century as defenders of the collapsing Abbasid caliphate. Beginning in the 11th century, Christians in Jerusalem were progressively being persecuted by the Islamic rulers of the region particularly when control of the holy city passed from the comparatively benevolent Egyptians to the Seljuk Turks in 1071, prompting the Byzantine emperor to turn to the West for assistance in producing a crusade to support the Eastern Christians and to regain the holy lands.