What is the "iron curtain"?

Winston Churchill first mentioned the word "iron curtain" during his speech on March 5, 1946: "From Stettin in the Baltic to Trieste in the Adriatic, an iron curtain has descended across the Continent. Behind that line lie all the capitals of the ancient states of Central and Eastern Europe. Warsaw, Berlin, Prague, Vienna, Budapest, Belgrade, Bucharest and Sofia; all these famous cities and the populations around them lie in what I must call the Soviet sphere, and all are subject, in one form or another, not only to Soviet influence but to a very high and in some cases increasing measure of control from Moscow." In Churchill's mind, the "iron curtain" defines the boundary which separates western European countries under US influence and eastern European countries under Soviet Control. This was further solidified by the creation of the Berlin wall and the Soviet supported Communist governments that ruled over eastern European countries throughout the cold war. After the collapse of Soviet Union, the iron curtain disappeared as many eastern European countries topped their Communist governments and joined the Western sphere of influence.