The Council of Ephesus was held in Ephesus, Asia Minor in 431 under Emperor Theodosius II, grandson of Theodosius the Great. Approximately 200 Bishops were present, though procedings began in haste before the arrival of the bishops from the west. The procedings were conducted in a heated atmosphere of confrontation and recriminations. It was the Third Ecumenical Council. It was chiefly concerned with Nestorianism.
According to the Council, Nestorianism overemphasized the human nature of Jesus at the expense of the divine. The Council denounced Patriarch Nestorius' teaching as erroneous. Nestorius taught that the Virgin Mary gave birth to a man, Jesus Christ, not God, the "Logos" ("The Word", Son of God). The Logos only dwelled in Christ, as in a Temple (Christ, therefore, was only Theophoros: The "Bearer of God".) Consequently, Virgin Mary should be called "Christotokos," Mother of Christ and not "Theotokos, "Mother of God." Hence, the name, "Christological controversies".
The Council decreed that Jesus was one person, not two separate "people": complete God and complete man, with a rational soul and body. The Virgin Mary is "Theotokos" because she gave birth not to man but to God as a man. The union of the two natures of Christ took place in such a fashion that one did not disturb the other.
The Council also declared the text of the Nicene Creed decreed at the First and Second Ecumenical Councils to be complete and forbade any additional change (addition or deletion) to it. In addition, it condemned Pelagianism.
Catholic Encyclopedia: (http://www.newadvent.org/cathen/05491a.htm) Ephesus, Council of; report of proceeding from the Catholic POV
Michael J. Svigel, "The Phantom Heresy:Did the Council of Ephesus (431) Condemn Chiliasm?" (http://www.bible.org/docs/soapbox/st-essay/phantomheresy.htm)
Councils are legally convened assemblies of ecclesiastical dignitaries and theological experts for the purpose of discussing and regulating matters of church doctrine and discipline.
EcumenicalCouncils are those to which the bishops, and others entitled to vote, are convoked from the whole world (oikoumene) under the presidency of the pope or his legates, and the decrees of which, having received papal confirmation, bind all Christians.
The Councils of Constance and of Basle affirmed with great emphasis that an Ecumenicalcouncil is superior in authority to the pope, and French theologians have adopted that proposition as one of the famous four Gallican Liberties.