Manhattan is the oldest and the most densely populated of the five boroughs of New York City. Located primarily on Manhattan Island at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York. The borough and county consist of Manhattan Island and several small adjacent islands: Roosevelt Island, Randall's Island, Wards Island, Governors Island, Liberty Island, part of Ellis Island,  Mill Rock, and U Thant Island; as well as Marble Hill, a very small area on the mainland bordering the Bronx. The original city of New York began at the southern end of Manhattan, expanded northwards, and then between 1874 and 1898, annexed land from surrounding counties.

The County of New York is the most densely populated county in the United States, and one of the most densely populated areas in the world, with a 2008 population of 1,634,795  living in a land area of 22.96 square miles (59.47 km²), or 71,201 residents per square mile (27,485/km²). It is also one of the wealthiest counties in the United States, with a 2005 personal income per capita above $100,000. Manhattan is the third-largest of New York's five boroughs in population, and its smallest borough in size.

Manhattan is a major commercial, financial, and cultural center of both the United States and the world. Anchored by Wall Street in Lower Manhattan, New York City vies with the City of London as the financial capital of the world^[  and is home of both the New York Stock Exchange and NASDAQ. Many major radio, television, and telecommunications companies in the United States are based here, as well as many news, magazine, book, and other media publishers.

Manhattan has many famous landmarks, tourist attractions, museums, and universities. It is also home to the headquarters of the United Nations. It is the center of New York City and the New York metropolitan region, hosting the seat of city government and a large portion of the area's employment, business, and entertainment activities. As a result, residents of New York City's other boroughs such as Brooklyn and Queens often refer to a trip to Manhattan as "going to the city",  despite the comparable populations between those boroughs.
Source: http://en.wikipedia.org/wiki/Manhattan