Protestantism refers to any of the Christian religious groups, of Western European origin, that broke with the Roman Catholic Church as a result of the influence of Martin Luther, founder of Lutheranism, and John Calvin, founder of Calvinism. Some Roman Catholics label any Western non-Catholic group as Protestant, even if the sect did not arise from Luther's theology (e.g. Anglican, Unitarian...) And some Protestants label Catholics non-Christian, often referring to "Catholics and Christians" (the latter term meaning Protestants and excluding the Orthodox tradition from consideration).
Protestants generally trace their separation from the the Roman Catholic church to the 1500's. This is the time the most successful reformers effected a permanent, substantial break with the church. Luther nailed his 95 theses on the door of the church at Wittenberg in 1517, and Calvin was active a generation after him. Other reformers included Ulrich Zwingli. John Wyclif, and Jan Hus were active in the 14th century and prefigured the Reformation.
Protestantism's major theological differences with the Catholics include the belief in the sufficiency of faith alone for salvation, the figurative (rather than real) presence of Christ in the bread and wine of communion, the lack of need for formal confession to a priest followed by acts of penitence, and the rule of the pope over the church. Generally speaking, Protestants thought and think that it is above all most important that one have a personal relationship with the divine, unmediated by a priesthood or church. It is uncommon for protestants to believe in the existence of purgatory.
Protestants refer to particular protestantism sects as denominations to imply that they are differently named parts of the whole church, although some denominations are less accepting of others and some are so unorthodox as to be questioned by most. All denominations consider all others to have some points of doctrine wrong. These theological differences are sometimes very small. Many denominations do not consider that the points of doctrine which other denominations have wrong as important enough to keep followers of the other denomination eternally separated from God or Heaven. The actual number of distinct denominations is hard to calculate, but has been estimated to be in the tens of thousands.
Protestant families of denominations:
Other well-known Protestants: