GNP vs GDP

Gross national product (GNP) is the market value of all the products and services produced in one year by labour and property supplied by the citizens of a country.

Gross Domestic Product (GDP), which defines production based on the geographical location of production, GNP allocates production based on location of ownership.

Comparisons of national wealth are also frequently made on the basis of purchasing power parity (PPP), to adjust for differences in the cost of living in different countries. PPP largely removes the exchange rate problem, but has its own drawbacks; it does not reflect the value of economic output in international trade, and it also requires more estimation than nominal GDP.

The United States is the world's largest national economy with a GDP of approximately $16.8 trillion

Sources : http://knoema.com/nwnfkne/world-gdp-ranking-2014-data-and-charts