This tool allows you to determine the area of a polygon.  Click on this tool, and then proceed to define the perimeter of a polygon by clicking on the corners of the polygon (either in clockwise order or counter-clockwise order).  The web site assumes that there is a straight line between the last point you clicked and the first point you clicked.  As you click on the corners, the area of the polygon will appear below the map, shown in square feet, acres, and square miles.