In the 9th century AD, two Greek missionaries, St. Cyril and St. Methodius, were sent to the 1st Bulgarian Empire and there developed the Cyrillic script, which was used in church books and liturgies created for use by the local inhabitants. Cyrillic was originally a hybrid of the Greek alphabet and the Glagolitic script already in use. Cyrillic eventually displaced Glagolitic and is currently used by 225 million people who are native speakers of Bulgarian, Russian, Serbian, Macedonian, Tatar, Ukrainian and many other languages.

More Info: en.wikipedia.org