After the beginning of World War II, Nazis began ordering all Jews to live within certain, very specific, areas of big cities, called ghettos.
Jews were forced out of their homes and moved into smaller apartments, often shared with other families.
Some ghettos started out as "open," which meant that Jews could leave the area during the daytime but often had to be back within the ghetto by a curfew. Later, all ghettos became "closed," which meant that Jews were trapped within the confines of the ghetto and not allowed to leave.
A few of the major ghettos were located in the cities of Bialystok, Kovno, Lodz, Minsk, Riga, Vilna, and Warsaw.
The largest ghetto was in Warsaw, with its highest population reaching 445,000 in March 1941.
In most ghettos, Nazis ordered the Jews to establish a Judenrat (a Jewish council) to both administer Nazi demands and to regulate the internal life of the ghetto.
Nazis would then order deportations from the ghettos. In some of the large ghettos, 1,000 people per day were loaded up in trains and sent to either a concentration camp or a death camp.
To get them to cooperate, the Nazis told the Jews they were being transported to another place for labor.
When the Nazis decided to kill the remaining Jews in a ghetto, they would "liquidate" a ghetto by boarding the last Jews in the ghetto on trains.
When the Nazis attempted to liquidate the Warsaw Ghetto on April 13, 1943, the remaining Jews fought back in what has become known as the Warsaw Ghetto Uprising. The Jewish resistance fighters held out against the entire Nazi regime for 28 days -- longer than many European countries had been able to withstand Nazi conquest.