Eastern Orthodox Church
The Eastern Orthodox Church is a group of national Churches (Russian Orthodox, Greek Orthodox, Serbian Orthodox, etc) which separated from Rome in the Great Schism, formally in 1054 but in practice going back much earlier. They accept the validity of the first seven ecumenical councils. The Eastern Orthodox do not accept the Petrine Primacy. The Ecumenical Patriarch of Constantinople is their nominal leader, but he has no direct authority over the other Eastern bishops like the Pope does in the West.
The beliefs and practices of the Eastern Orthodox are similar to Catholics in most respects. However, while reciting the Nicene Creed, the East confesses the Holy Spirit proceeds only from the Father, not from the Father and the Son like Catholics believe. Eastern Orthodox perform the Sign of the Cross right to left, not left to right. Mass is attended while standing up, not seated. EO priests may be married, but not bishops.