Reformation
The Reformation, also known as the Protestant Reformation, was a major theological movement within the Western Catholic Church, that opposed many doctrines of the Papacy and other abuses which had crept into the Church during the Middle Ages. It is traditionally dated to have begun with Martin Luther and the publication of the Ninety-Five Theses in 1517. However, many prior Reformers preceded Martin Luther during the Middle Ages, including John Wycliffe and Jan Hus, who anticipated the critiques of Luther and the Lutherans.
Doctrine
The Protestant Reformers began to coalesce around a few core doctrines upon which they all agreed. This being, the Supremacy of the Holy Scriptures in all matters of faith (often called Sola Scriptura), the fallibility of church authorities, Justification by Faith Alone (often called Sola Fide), and opposition to the doctrines of Papal Indulgences, Purgatory, the Treasury of Merit, Transubstantiation, and the Primacy and Infallibility of the Pope of Rome. The Reformers also were united in teaching that sinners were saved by grace alone (Sola Gratia), meaning that the elect did not earn their salvation by meriting it.
However, eventually the Reformers were divided on the Sacraments. Ulrich Zwingli had begun a parallel Reformation in Switzerland, that shared many of the same ideals as Martin Luther's faction. In 1529, Luther and Zwingli met in Marburg, Germany to attempt to unite. On almost every article of dispute they agreed, except for on the Real Presence of Christ in the Eucharist. Martin Luther and his faction taught a strict substantial presence of Christ in the consecrated elements, while the Zwinglians denied this substantial presence. While Zwingli agreed to disagree, Luther would not fellowship with people who denied the substantial Real Presence of Christ in the elements on the Altar. This was the beginning of the split between the Evangelicals (also called Lutherans) and the Reformed (called Zwinglians at the time, but later called Calvinists).