Short answers, pulled from the story.
Girolamo Savonarola was an Italian Dominican friar born on the 21st of September 1452 in Ferrara who became the de facto ruler of Florence from 1494 to 1498. He is significant as a religious reformer who expelled the Medici, reshaped Florence's constitution, clashed openly with Pope Alexander VI, and was later regarded by Martin Luther as a forerunner of the Protestant Reformation.
The bonfires of the vanities were public burnings of secular artworks, books, and objects deemed morally corrupting, carried out as part of Savonarola's moralistic campaign in Florence. His lieutenant Fra Silvestro Maruffi organised boys and young men to patrol the streets and confiscate items for destruction, while Savonarola himself replaced the era's Carnival songs with devotional music composed with the poet Girolamo Benivieni.
Pope Alexander VI excommunicated Savonarola on the 12th of May 1497 for heresy and sedition. The break was precipitated by Florence's refusal to join the pope's Holy League against France in 1495, which Alexander blamed on Savonarola's influence, and then by Savonarola's repeated defiance of a ban on preaching and his increasingly inflammatory attacks on the papal court.
Savonarola was hanged and his body burned in the main square of Florence on the 23rd of May 1498, along with two of his fellow friars, Fra Domenico da Pescia and Fra Silvestro Maruffi. Church and civil authorities had condemned the three as heretics and schismatics following a trial. Their ashes were scattered in the Arno river to prevent followers from collecting relics.
Martin Luther read Savonarola's writings and praised him as a martyr and forerunner whose ideas on faith and grace anticipated Luther's doctrine of justification by faith alone. A statue of Savonarola was erected in Wittenberg, Luther's hometown. In France, Savonarola was translated and regarded as a precursor of Huguenot, or evangelical, reform.
Savonarola's devotees, known as the Piagnoni, kept his cause of republican freedom and religious reform alive well into the following century. The friars of San Marco preserved his sermons and writings and fostered a cult of the three martyrs. The movement was suppressed after the Medici returned to Florence in 1512, though it was briefly revived in 1527 when the Medici were again expelled.