Ellis Island was at one time the main entry facility for immigrants entering the United States. The federal immigration station opened on January 1, 1892 and was closed on November 12, 1954, but not before 12 million immigrants were inspected there by the US Bureau of Immigration. There are unsubstantiated estimates for immigrants processed there as high as 20 million. Ellis Island was one of 30 processing stations opened by the federal government. It was the major processing station for third class immigrants entering the United States in 1892; it processed 70% of all immigrants at the time. Wealthy immigrants that traveled first class and second class would get automatic entry into the United States. Those who did not travel first or second class had to pass a six second physical examination.
Today the main building houses a museum in addition to being a historic site.

Sources, related links:
Ellis Island Immigration Museum - official web site
Ellis Island - Free Port of New York Passenger Records Search
Official web site - Ellis Island National Monument