Protestantism is generally regarded as a movement begun by Martin Luther in 1517. While clearly not wishing to separate himself from the Catholic Church, he spoke of reforming certain doctrines and teachings regarding the sale of indulgences. These reforms were written up in 95 theses that were then nailed to the door of the Castle Church in Wittenberg.
As Luther's teachings spread in the convening 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"movement" led to the temporary injunction of the ban as expressed by the Diet of Speyer in 1526, but was later rescinded in 1529.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, including: