Westchester County is located to the north of New York City. People who live in NYC have an annoying habit of referring to Westchester as "upstate New York" even when we are actually in southern New York. The county is about 457 square miles. It is bordered on the west by the Hudson River
The area was first settled in the 1600's by the Dutch, and later taken over by the English. Stone walls from the time when it was all farmland can still be found.
The county is often thought of as filled with rich suburbanites, and while this is true for towns like Scarsdale and Chappaqua (where the Clintons live), there are also areas that are more urban and do have ghettos and poor people, like Yonkers and Peekskill.
The seat of government is in White Plains and the county government's website is westchestergov.com, possibly the most annoying URL i know as it includes a gov at the end but is really a .com.