The Great Schism was the crisis of The Papacy that split Western Christianity from 1378 until 1417. By the time of Pope Gregory XI permanently returned The Curia to Rome, The Pope was widely regarded as a “tool of French power”. Such feeling lead Florence (Italy) and it's Allies to raise a revolt in The Papal States. With Pope Gregory's death in 1378 (without a peace), a Conclave was called for a new Pope. The Conclave was made up of predominately French Cardinals. Obviously, a French Pope was chosen (with which the outnumbered Italian Cardinals were forced to agree).
So Urban VI was installed. His French Cardinals were sent to The French See in Avingon (France), to serve him in an Administrative capacity. While in Avingon, the Cardinals lives as Royal Princes. “Requested” to come to Avingon by his Cardinals, Pope Urban refused and stated that he was going to reform The Church. Not to be undone, (and no doubt fearing the loss of their “station” in life) – the Cardinals declared that The Pope's election to be coerced and invalid. Choosing a “new” Pope (Clement VII), and setting up The Holy See “officially” in France, a civil war was started.
Rome considered this to be an “Antipope” and all who followed him or his Cardinals (or any persons installed as Clergy) – invalid and excommunicate (and visa verse). With each passing new Papal installment 2 men would be placed on “The Chair of Peter”. Sometimes referred to as “The Avignon Captivity”, is was attempted to have been resolve by many people.
By 1414, a final Council (of Constance), was called to repair the split. Resolved, in 1417 Pope Martin V became the only head of The Church. The Great Schism did not repair The Church however. Many saw it at fatally weakening The Papacy. Further Conciliarism would become official Doctrine. Finally many of The Church's gained concessions (with European Powers) over the past 40 years – were gone.