What a is a ghetto?

“During World War II, ghettos were city districts (often enclosed) in which the Germans concentrated the municipal and sometimes regional Jewish population and forced them to live under miserable conditions. Ghettos isolated Jews by separating Jewish communities from the non-Jewish population and from other Jewish communities. The Germans established at least 1,000 ghettos in German-occupied and annexed Poland and the Soviet Union alone. German occupation authorities established the first ghetto in Poland in Piotrków Trybunalski in October 1939.
There were three types of ghettos: closed ghettos, open ghettos, and destruction ghettos.
The largest ghetto in Poland was the Warsaw ghetto, where over 400,000 Jews were crowded into an area of 1.3 square miles. Other major ghettos were established in the cities of Lodz, Krakow, Bialystok, Lvov, Lublin, Vilna, Kovno, Czestochowa, and Minsk. Tens of thousands of western European Jews were also deported to ghettos in the east.”
(http://www.ushmm.org/wlc/article.php?lang=en&ModuleId=10005059)