Real versus nominal value (economics)
Gross domestic product (GDP) is a measure of aggregate output. Nominal GDP in a particular period reflects prices which were current at the time, whereas real GDP compensates for inflation. Price indices and the U.S. National Income and Product Accounts are constructed from bundles of commodities and their respective prices. In the case of GDP, a suitable price index is the GDP price index. In the U.S. National Income and Product Accounts, nominal GDP is called "GDP in current dollars" (that is, in prices current for each designated year), and real GDP is called "GDP in [base-year] dollars" (that is, in dollars that can purchase the same quantity of commodities as in the base year).
Net domestic product
If the country is not able to replace the capital stock lost through depreciation, then GDP will fall. In addition, a growing gap between GDP and NDP indicates increasing obsolescence of capital goods, while a narrowing gap means that the condition of capital stock in the country is improving.
It reduces the value of capital that is why it is separated from GDP to get NDP.