The Byzantine empire was part of the Roman empire. It was founded in 395 AD by Emperor Constantine the Great and lasted until 1453, when the capital was captured by the Ottomans.
One of the most important emperors was Justinian the Great, the 19th emperor of the Byzantine Empire, who tried to reconquer all the territory Rome lost when it was destroyed. He conquered most of it, but could not achieve his dreams. He wrote the laws of the kingdom and built the Hagia Sophia, a gigantic church in Constantinople.
The Byzantine Empire was weakened in the Fourth Crusade period when Alexios Angelos attacked Constantinople with the crusaders he commanded. They destroyed many buildings and put Alexios on the throne. Unluckily for him, the people hated their new emperor and threw him out. After this, the Ottomans started attacking Byzantine territory and conquered the capital, Constantinople in 1453.
In the Byzantine Empire, icons were worshiped, but in other places, they were not. This led to iconoclasm, which was when the western church tried to stop the Eastern church from worshiping icons. This did not work, because the emperor consented to the worship of icons.