Staten Island is one of New York City
's five boroughs. Its population is approx. 500,000 (less than any other borough).
It is most well-known for the Fresh Kills Landfill (yes, that's its name) and the Staten Island Ferry. It's also the starting point for the New York City Marathon.
Staten Island is connected to Brooklyn by the Verrazano Narrows Bridge and to New Jersey by the Goethals Bridge, Bayonne Bridge, and the Outerbridge Crossing.
While part of New York City, Staten Island is more like suburban New Jersey or Long Island. It is highly residential and most people live in their own houses rather than apartments. Socially "in" things include bowling, going to the movies and, of course, going to the Staten Island Mall. (What would suburbia be without a mall?)
Residents of Staten Island have often had different views from the rest of New York City. During the American Revolution era, Staten Island had strong British support, while the rest of New York City was largely patriots. Staten Island is strongly Republican, while the rest of New York City is highly Democrat. In fact, without Staten Island, Dem. David Dinkins would've defeated Rep. Rudolph Giuliani in a mayoral election in the 1990s. (Giuliani won Staten Island by more votes than Dinkins won the rest of the city, putting him over the top).
There was a strong movement in the 1990s for Staten Island to secede from New York City, but that has subsided in the last few years.