National income is defined as the total value of final goods and services produced in a country in one year. It can be measured using Gross Domestic Product (GDP), Gross National Product (GNP), Net Domestic Product (NDP), and Net National Product (NNP). GDP is the total market value of goods and services produced domestically, while GNP includes net income from abroad. NDP is GDP minus depreciation, and NNP is GNP minus depreciation. Per capita income divides national income by population for average income.