The Byzantine Empire (~500-1500) was an empire that existed in the eastern part of the Mediterranean region. It comprised the remnants of the Eastern Roman Empire, which in contrast to the Western Roman Empire was mainly Greek speaking and Eastern Orthodox in religion.
After several catastrophic years and defeats to barbarians the eastern Romans recovered somewhat, reconquering lands as far as North Africa, the Balkans, the Middle East, and the Italian peninsula. The main Byzantine heartland was in Greece and Asia Minor, and the capital was Constantinople.
They were weakened by the fourth Crusade sack of Constantinople in the 13th century, and finished off by the Ottoman Turks in the 15th century, with Constantinople falling in one of the most epic sieges ever.
Byzantines had advanced technology for the time and helped to keep Roman knowledge from disappearing during the Middle Ages. They had Greek Fire, a naval weapon similar to napalm, used to ignite opposing ships.