Protestantism is generally regarded as a movement begun by Martin Luther, a German monk, in 1517. While clearly not wishing to separate himself from the Catholic Church, he spoke of reforming certain doctrines and teachings of the church. These reforms were written up in 95 theses, which Luther titled "Disputation of Martin Luther on the Power and Efficacy of Indulgences" and mailed with a letter to the Archbishop of Mainz and Magdeburg, Albrecht. He is also said to have nailed the theses to the door of the Castle Church in Wittenberg on the same day. According to Luther, the sale of indulgences was nothing more than a way for the church to take money from the poor. The practice was started by Pope Leo X in order to raise money to rebuild Saint Peter's Basilica in Rome. Luther's understanding of the Bible was that faith alone (sola fide) was required for salvation, whereas the church taught that good works were also necessary, and could be obtained by purchasing indulgences as an act of good works.
As Luther's teachings spread in the intervening years and became more and more a source of frustration for the Church, certain princes and elite members in society joined in the fray supporting Luther. In 1521, as described by the Edict of Worms, Luther's teachings were outlawed and his death was implicitly sanctioned by the Church. The dissenting princes protested and sought to have the Church respect an individual's right to believe as he pleased. This protestant "movement" led to the temporary injunction of the ban as expressed by the Diet of Speyer in 1526, but was later rescinded in 1529.
As the years passed, Protestantism generally came to define any individual or group's actions that undermined the authority of the Catholic Church and any people who stated their desire for a formal separation from the Church.
Today, Protestantism is actually an umbrella term that includes many Christian denominations (estimates put the number at over 20,000), including: