1. The Gini coefficient marks inequality, usually in income or wealth, but it has other applications as well. When it is typically used, economists and policy makers would be interested in this, since it shows how unequitable a country is, and shows that there is a monetary divide.
2. The census bureau will calculate it. And they derive it from the lorenz curve, and their data on households vs income. Which is taken every census year.
3.The 2008 Gini coefficient rating for the U.S was .466 which can be compared to canada or most countries in europe which sit in the 20's and 30's. http://en.wikipedia.org/wiki/File:Gini_Coefficient_World_CIA_Report_2009.png
Source http://www.census.gov/prod/2009pubs/p60-236.pdf and http://en.wikipedia.org/wiki/Gini_coefficient#Calculation