Martin Luther never did things by halves. He promised to devote his life to God's service if the Lord would save him from a lightning storm; he reportedly wrestled physically with the Devil in an effort to overcome his own sinfulness; he studied so hard that he forgot to change his bedsheets, resulting in a moldy bed.
After the Reformation began, he became even more larger-than-life. To illustrate the truth that God's Word is for all people, he unchained a man-sized church Bible and carried it outside on his back like Samson carrying away the pagan gates. He claimed the thought of preaching each Sunday made him physically ill with its implications of responsibility. On October 31, 1517, he nailed 95 Theses on the door of the Wittenberg Church, and openly accused the Pope of heresy.
None of this was for personal profit or glory, however. A lot of people do fairly ridiculous things to get attention or an entry in the history books; Luther was motivated by a love of Christ and the Gospel, and a desire for those things to be known and loved by others. What he initiated was one of the most important and dramatic periods in the history of the world: the Protestant Reformation.
What set him off (and what most of the Theses are about) was clerical corruption. The Catholic Church oversaw the sell of Indulgences, purchasable scraps of paper assuring the buyer (or a loved one) a reduction of temporal or eternal punishment. There was rampant greed, immorality, extortion, abuse of power, etc., as well as doctrinal heresy; Luther stood against them with only the Bible and orthodox doctrine for weapons.
Ultimately, many of the questions his stance raised concerned authority and its origins. Is the Church the arbiter of authority, or is it Christ alone? if Christ alone, how is authority practically applied? what authority do clergy maintain? is theirs the earthly manifestation of Christ's primary and heavenly authority? if so, what happens when they overstep their bounds or behave in disaccord with the Gospel's dictates?
Catholic doctrine stated that the Pope, cardinals, prelates, and priests were the hands of Christ on earth, guiding and leading the Church and acting as its vicar (the Pope) and watchmen (everyone else). What the Reformers found upon examination of Scripture was that every believer is Christ's representative, a priest in His name, and that it is Christ who maintains authority of His Church.
That didn't mean, of course, that humans had no authority over one another. But, like a married couple, all believers ought to submit to one another, though leaders are to be appointed to maintain order and guidance. This rejection of centralized power (the Papacy) was revolutionary, and Europe never fully recovered.
Others soon joined Luther's mission of reform: John Calvin, John Knox, Martin Bucer, Ulrich Zwingli, and Theodore Beza, to name a few. Their work wasn't simply to tear down a defunct system, it was to rediscover and uphold the beauty of the Christian Gospel as revealed in God's Word, and untainted by human tradition and interpretation.
The Protestant Reformation also led to reform within the Catholic Church. Godly men like Erasmus, not convinced by the arguments of Luther and Calvin yet aware that change was necessary, led the Counter Reformation with the purpose of returning the Catholic Church to its roots. These men later became known as Christian humanists, and their efforts in some ways led to the Enlightenment thinking of later centuries.
If you're Christian and aren't Catholic or Orthodox, you're Protestant. Many Protestants have abandoned the Reformed doctrines of men like Calvin and Knox, which is one of the dangers inherent in de-centralizing church authority. However, the blessings of pure doctrine and Christ-centered worship far outweigh the possible pitfalls of apostasy, and we thank God humbly and openly for sending men in His name to redirect His people toward truth and faithfulness.