A salt water estuary which separates Long Island from Connecticut and most other points north. (The island is bordered on the west by the East River, and on the south and east by the Atlantic Ocean.)
Long Island Sound is relatively young, having been formed about 11,000 years ago by glaciers during the Ice Age. It is still not quite "stable" -- the water level continues to rise every year, making erosion a fairly difficult problem for beachfront properties. Still, a number of beaches, including Tobay Beach in Oyster Bay, are open for residents of their respective towns.
The Sound is also home to various fish and shellfish, including clams and oysters. Lately, depopulation of many species has led to lower yields and more strict controls on catches. This has hurt the local seafood trade, and has generally resulted in rising prices.
Although there are no bridges which currently cross the Sound, there are ferries which can bring you from Orient Point, New York to New London, Connecticut. This saves a slight amount of time and aggravation for people driving from the East End of Long Island up to, say, Boston.