Nominal GDP is the value of all goods and services produced in a country, without adjustments for inflation.
GDP is the market value of all final goods and services produced within a country in a given period of time, and is usually expressed quarterly. Nominal GDP is primarily used to compare quarters in the same year, and does not contain an inflation adjustment as with Real GDP, which is more useful for comparisons across years.
The equation used to calculate GDP is something like this: Consumption (Goods and Services Purchased) + Government Expenditures (for infrastructure like Defense, Roads and Schools) + Investment (capital projects + Net Exports (minus Imports)). Gross National Product is a different measurement, which considers all of the goods and services produced by citizens of a county.
So it is essentially GDP plus income from foreign operations which benefit US citizens, minus the output of foreign-owned companies within the US borders.