Short answers, pulled from the story.
The Counter-Reformation is frequently dated from the opening of the Council of Trent in 1545 to the political conclusion of the European wars of religion in 1648, though scholars consider this periodization controversial. Joseph II's Patent of Toleration in 1781 is also regarded by some as the end of the political Counter-Reformation.
The Council of Trent met sporadically from the 13th of December 1545 to the 4th of December 1563 and was the institutional heart of the Counter-Reformation. It reaffirmed Catholic doctrine including transubstantiation and the seven sacraments, rejected Protestant positions on faith alone, commissioned the Roman Catechism, standardized the Mass form in 1570, and gave bishops greater authority to supervise religious life and combat absenteeism.
The term is a translation of Gegenreformation, coined by German historians in the late eighteenth century as a negative concept describing Catholic repressions. Leopold von Ranke helped popularize it through the mid-nineteenth century. Catholic historians largely shunned it, preferring "Catholic Reformation," a phrase introduced by Protestant historian Wilhelm Maurenbrecher in 1880.
The Jesuits, canonically recognized in 1540, were considered the most effective of the new Catholic orders. Organized along military lines and shaped by Ignatius of Loyola's Spiritual Exercises, they worked in rural parishes, participated in missionary expansion in the Americas and Asia, and contributed to the Counter-Reformation Church along lines harmonized with Rome's authority.
The Council of Trent did not ban polyphony. The 22nd session in 1562 required that secular elements be kept out of Mass music while leaving polyphony implicitly allowed. The widely repeated legend that composer Giovanni Pierluigi da Palestrina saved polyphony by performing before the council is unfounded; the saviour-myth was first spread by an account from Aggazzari and Banchieri in 1609.
The Council of Trent's 1563 decrees instructed that all lasciviousness be avoided in religious images and that no unusual image be placed in a church without the bishop's approval. These decrees, amplified by books from Molanus, Charles Borromeo, and Cardinal Gabriele Paleotti, effectively prohibited much traditional iconography, classical pagan elements, and almost all nudity in religious art. The great medievalist Emile Male called the result "the death of medieval art."