Short answers, pulled from the story.
The Western Schism was a split within the Catholic Church that lasted from the 20th of September 1378 to the 11th of November 1417. During this period, competing claimants in Rome and Avignon simultaneously claimed to be the true pope, and a third line of Pisan claimants joined the dispute in 1409.
The schism began after the death of Gregory XI and the disputed election of Urban VI. A group of French cardinals declared Urban VI's election invalid on the grounds of intimidation and violence, then elected Clement VII at Fondi on the 20th of September 1378, creating two rival popes.
The schism was resolved at the Council of Constance, convened in 1414. The council secured the resignation of Gregory XII, removed John XXIII, and excommunicated Benedict XIII on the 27th of July 1417. Pope Martin V was then elected, ending the forty-year division.
At its peak there were three simultaneous claimants. Two lines, Roman and Avignon, existed from 1378, and a third Pisan line was added in 1409 when the Council of Pisa elected Alexander V in an attempt to resolve the dispute.
Clement VII, the Avignon claimant, secured the support of France, Castile, Aragon, Navarre, Poland, Lithuania, Hungary, Scotland, and most of Italy except Naples and Savoy. Owain Glyndwr's Welsh rebels also recognized the Avignonian line.
The schism accelerated Conciliarism, a movement holding that a general council ranks above the pope. Scholars also link it to a marked decline in church discipline and authority. Many historians view it as a contributing factor in the Protestant Reformation of the 16th century.