What countries are considered "Eastern European"?

The eastern ones, and which those might be could depend upon whom you ask. Czechs, for example, generally know that the Czech Republic is a Middle European country. People further west might term it eastern.

Certainly Russia, Belarus, Ukraine, Moldavia, Romania, Bulgaria, Serbia. Maybe Greece, Croatia, Bosnia-Herzegovina, the Baltic States, Poland and Slovakia. It depends on what definition of Eastern Europe you use. Some possibilities are below.

1) Any country that had an Eastern Orthodox form of Christianity as it's dominant religion.

2) Any country that was a part of the Soviet Bloc.

3) East of Central Europe (Germany, Austria, Czech Republic)

Pretty much everything east of Germany, Italy, Austria, and Denmark, especially if they used to be Commies.