Cyril and Methodius were brothers, Methodius (815 - 885AD) and Cyril, whose real name was Constantine (826 - 869AD) they were sons of a senator in Thessalonica. Methodius became governor of a province and Cyril was a teacher of distinction at the imperial university in Constantinople and later librarian at Santa Sophia. In their youth the brothers learnt a number of Slavonic languages, none of which were written at this time. They both became priests.
In about 863AD Rostislav, ruler of Moravia¹, asked the Emperor for the brothers to come and teach his people in the vernacular, and they accepted with great enthusiasm. They set up the Christian church in Moravia and translated the Bible and liturgical works into Slavonic language. As there was no alphabet Cyril invented the Glagolithic one, which later developed into the Cyrillic one used in Slav lands and subsequently in Russia. It was based on Greek and symbols were added. The brothers became know as Arc Apostles of the Southern Slavs.
Rostislav wanted his people to be linked to Constantinople, but that brought conflict between the brothers and the German missionaries, who refused to ordain their followers. The brothers went to Rome to get papal support, taking with them the relics of St Clement in 869AD, and they were welcomed with great acclaim. Cyril became a monk, and then died.
Pope Adrian made Methodius Bishop of Pannonia and sent him back. Conflict continued and Methodius was imprisoned until Pope John VIII made the German bishops release him. In 879AD he was summoned to Rome to answer to the bishops' accusations of heresy, but was cleared and sent back as Archbishop of Moravia.
Methodius died in Velehrad, and the bishops then expelled his main followers to what is now Bulgaria.
The brothers are considered to have founded Slavonic literature, and are greatly revered by Czechs, Croats, Serbs and Bulgars. They were also the first churchmen to use the vernacular for religious services. Pope John Paul II declared them patron saints of Europe with St Benedict
¹Moravia is a historical region in Central Europe in the east of the Czech Republic, together with Bohemia and Silesia one of the former Czech lands. It takes its name from the Morava River which rises in the northwest of the region. Moravia's largest cities is Brno, its also historical capital. (See Wikipedia for full geographic details)