Gross National Product is the total value of all final goods and services produced by a country's factors of production and sold on the market in a given time period. Nominal GNP measures the value of output during a given year using the prices prevailing during that year. Over time, the general level of prices rises due to inflation, leading to an increase in nominal GNP even if the volume of goods and services produced is unchanged. GNP does not include goods produced on a subsistence level, i.e. farmers who eat their own products do not have their crops included in the GNP.
Real GNP measures the value of output in two or more different years by valuing the goods and services adjusted for inflation. For example, if both the "nominal GNP" and price level doubled between 1995 and 2005, the "real GNP" would remain the same. For year over year GNP growth, "real GNP" is usually used because it gives a more accurate view of the economy. It also has nothing to do with the population.