This calculator computes distance between two points on Earth. It uses Vincenty formula.
You can read more here: http://en.wikipedia.org/wiki/Great_circle_distance
Since Earth is not an ideal sphere, calculator uses mean radius value according to WGS 84, which can give 0.5% error, according to wikipedia.
By default calculator coordinates are of New York and San Francisco
First place latitude
First place longitude
Second place latitude
Second place longitude