Eastern Europe is a region of Europe that consist of Russian Federation (its European part), and occasionally Belarus, and Ukraine. These countries are about 50% of European landmass.
Eastern Europe is often confused or referred as a shortcut to in the media with the Eastern Bloc that consisted of the Soviet Union - especially Baltic states in Northern Europe(Latvia, Lithuania, Estonia), and those that weren't a part of it - Balkan states in Southern Europe (Albania, Bulgaria, Moldova, Romania, Yugoslavia), and Central European nations (Czechoslovakia, East Germany, Hungary and Poland).
Czechoslovakia split into the Czech Republic and Slovakia in 1993, while Yugoslavia has splintered into several countries in early 1990s. East Germany has been reunited with West Germany to form Germany. The Soviet Union broke up in 1990s into Russia, Georgia, Ukraine, Moldova, Belarus, Armenia, Azerbaijan, Kazakhstan, Uzbekistan, Turkmenistan, Kyrgyzstan, Tajikistan, Estonia, Latvia and Lithuania.
Bulgaria, Czech Republic, Estonia, Germany, Hungary, Latvia, Lithuania, Poland, Romania, Slovakia and Slovenia are members of the European Union. Croatia will be the first state to join it in 2013.
Contemporary use ad developments
Borders of European regions have been shifted back to the pre-Cold War, hence European Russia is also equal to pre-Cold War terms. Relating to Central European states as Eastern Europe is seen in Europe as ignorance or a provocation revoking the Cold War divide of Europe.
It is a common mistake to label all Slavic or the former Warsaw Pact (also called the Eastern Bloc) states in the region as located in Eastern Europe.
East and West Germany were countries, so better to call it eastern and western Germany. Reunification is all but a thing of the past and seen in a more or less positive light by most there and in all of Central Europe so try to avoid labeling Germans by their recent past.