The Superdome is a very large domed stadium used for American-style football and other events.
President Bush had called the Louisiana governor early this morning to ask her to issue a mandatory evacuation order. However, several facilities, such as hospitals and hotels, are exempt because the people there have no practical way to evacuate the city. In addition to the Superdome, a number of other shelters have been set up around the area.
Over on the weather channel, the eye appears headed directly for the western edge of the city. Basically, New Orleans is in the bullseye of the storm, if not exactly dead center.
In addition, the hurricane is well over 100 miles in diameter, which means communities on both sides of New Orleans will be hit hard.