The Reformation was a schism within Western Christianity initiated by numerous religious reformers in late Mediaeval and early modern Europe. The time just before the Reformation was known as the Renaissance. The reformers were mostly disgusted with the level of corruption in the Roman Catholic Church and protested against this corruption. Therefore they were known as Protestants.
The Reformation began in Wittenberg, Saxony, where Martin Luther nailed his Ninety-Five Theses on the Power and Efficacy of Indulgences to the door of the Castle Church, in Wittenberg  Protestants are united in their refusal to accept the authority of the Pope and the Roman Catholic hierarchy. Apart from that Protestants vary enormously.
The most prominent Protestant movements are listed in the article on Protestantism.
Prior to the ReformationEdit
There has been opposition to the Roman Catholic Church and schism throughout its history. The Gnostic heresy was one of many early examples. Orthodoxy was another. Cartharism and Waldensianism, which started roughly at the same time as the Eastern Orthodox Church were yet further schisms against the Roman Catholics. German mysticism in the late Middle Ages and the Hussite movement which started round the 14th Century are seen as precursors to the Reformation.