Charles IV (born Wenceslas IV, not to be confused with his own son, Wenceslas IV) was king of Germany and Bohemia, emperor of the Holy Roman Empire, and the founder of the Czech state.
Charles was the son of the warlike John of Luxembourg, who was the first of the Luxembourg family to occupy the Bohemian throne; his mother was Eliska Przemyslova, daughter of Wenceslas III, who John married to cement his claim to the throne. He was sent to the French court at age 7 to be educated; it was there that he came to be called Charles. He married the sister of king Philip VI of France, and was taught by the future Pope Clement VI, before being called back to Luxembourg in 1330. His father wished him to administer the family holdings in northern Italy, which he did with such competence that his father became alarmed and dismissed him in 1335. The two were soon reconciled, however, and Charles was soon John's most trusted and competent administrator. A year later, Clement VI (now Pope) made Prague an archbishopric, at Charles' urging.
John of Luxembourg died in 1346 at the battle of Crecy, fighting alongside his brother Charles II of Alencon against Edward III, Black Prince of Wales. Charles IV was chosen by the electors of the church to be king of Germany, despite the fact that there was already a king of Germany - Louis IV, who had been excommunicated by the Pope in 1324. The two made preparations for war, but Louis died before the battle could be joined. The ravages of the Black Plague and a lack of fitting successors to Louis' reign ensured that by 1347 Charles had sole rulership of Germany. He was also crowned king of Bohemia in 1347 by the new archbishop in Prague, and swiftly moved to establish constitutional laws of succession for the Bohemian kingship.
Bohemia flourished under Charles' reign. In 1348, he established Charles University - the oldest university in Europe - at Prague, his imperial capital, and began the rebuilding of St. Vitus Cathedral. In 1362, he commissioned an 80m set of fortifications on Petrin hill (the highest of Prague's seven hills) to provide work for the unemployed poor of the city, who were hard-hit by the famine of 1360-1361. This fortification was known as the "Hunger Wall," and remnants of it can still be seen in modern Prague. Charles initiated numerous other construction projects, including Karlsteijn castle (where the imperial crown jewelry and the insignia of the Bohemian king were kept), and a beautiful bridge across the Vltava River that runs through Prague. This bridge is still in use and is a major attraction in modern Prague; it appears in the movie Mission: Impossible. He also brought new agricultural industries to the city, and had vinyards, orchards, and forests planted in the area.
Charles continued to build the Bohemian state throughout his rule, as well. In 1355 he received the Iron Crown of Lombardy in Milan, and on Easter Sunday of the same year he was crowned Holy Roman emperor by the papal legate in Rome. The following year, he issued a law called the Golden Bull, in which (among other things) he decreed that henceforth the Holy Roman emperor would be elected by the majority vote of seven electors. This law completely excluded the papacy from the imperial succession, and fixed the constitutional form of the empire, which it would retain largely intact until its dissolution in 1806.
As Charles grew older, his thoughts turned to the succession of his son, Wenceslas IV. This he accomplished - mainly by bribing the German electors, and by securing the Pope's approval by promising not to ask the princes to elect Wenceslas during Charles' lifetime. The increased taxes Charles levied to pay for the bribes caused an uprising in the cities of Swabia, which he quelled in 1378 by offering concessions. He died later that year, and his body was interred in the crypt of the as-yet-unfinished St. Vitus Cathedral.
I used information from infoplease.com's entry on Charles IV, Patricia Hefner's "The Czechs and Moravians: a History," the Web Gallery of Art, and encyclopedia.com's entry on the Golden Bull to prepare this summary.