Short answers, pulled from the story.
Eastern Europe has no single agreed definition. Narrow definitions include Belarus, Russia, and Ukraine. Broader definitions add Moldova, Romania, some or all Balkan states, the Baltic states, the Visegrád Group countries, and the Caucasus nations of Armenia, Azerbaijan, and Georgia. Russia is the largest and most populous country in any definition of the region.
The Iron Curtain was the ideological and political divide that separated communist Eastern Europe from Western Europe during the Cold War between 1947 and 1989. Winston Churchill coined the phrase in his "Sinews of Peace" address on the 5th of March 1946, delivered at Westminster College in Fulton, Missouri. During this period, Eastern Europe was nearly synonymous with the Eastern Bloc and the Warsaw Pact.
Serfdom persisted in Eastern Europe well into the 19th century. Russia abolished serfdom in 1861, which marked a key turning point in its decline across the region. Former serfs were required to make annual cash payments to their former masters for decades after emancipation.
The fall of the Iron Curtain in 1989 triggered major political and economic restructuring. Germany reunified in 1990, and in 1991 the Soviet Union, COMECON, and the Warsaw Pact were all dissolved. Between 2004 and 2013, ten Eastern and Central European countries joined the European Union: the Czech Republic, Estonia, Latvia, Lithuania, Hungary, Poland, Slovenia, and Slovakia in 2004; Bulgaria and Romania in 2007; and Croatia in 2013.
The East-West Schism of 1054 formally divided Eastern Orthodox and Western Catholic Christianity, and it became a defining cultural boundary for Eastern Europe. After the split, much of the region developed shared identity through the Eastern Orthodox Church, the Church Slavonic language, and the Cyrillic alphabet. Eastern Christendom was not fully regarded as part of "Europe" in the cultural sense until the early 19th century.
Scholars have noted there are almost as many definitions of Eastern Europe as there are people who study it. The term is considered by some to be pejorative because of stereotypes associating the east with poverty and underdevelopment relative to Western Europe. The United Nations has described spatial identities like this as essentially social and cultural constructs, and the idea of Eastern Europe has been described as a "fuzzy" term in constant redefinition since the Enlightenment.