Nominal GDP is a measure of productivity of an economy and is the sum total of all the products and services generated by the economy in a given period.
Real GDP, on the other hand, is the measure of productivity corrected for inflation.
These two are related as:
GDP deflator = (nominal GDP/Real GDP) x 100
A value of more than 100 represents inflation, while a value of less than 100 refers to deflation (fall in prices) in the economy.
Because of inflation in the US economy over time, nominal GDP has risen faster than the real GDP. For example, a phone that someone bought for $100 in 2014 is available for $105 in 2015 because of 5% inflation. The nominal cost of the phone is $105, but the real cost (corrected for inflation) is still $100.
Nominal GDP also increases because of increased productivity over time (this will also increase real GDP), apart from the cost increase due to inflation (which will not affect the real GDP). And hence the nominal GDP of the US has risen much faster than the real GDP.