National income measures monetary value of all goods that were produced within the country. To measure national income there were generated three different accounts:
Total expenditure, total output of goods and services and total amount of income.

GDP
Gross Domestic Product measures the value of output produced within the domestic industries. However there are a lot of factories which are located abroad and if we add to GDP income from firms abroad owned by the particular country, so final output of this country will be called Gross National Product. So, GNP = GDP + Net Property Income from Abroad
There are three ways in calculating GDP:
1) Output method which takes into account adding the value for each stage in the production of goods. This method is usually used in manufacturing, construction, agriculture
2) Aggregate demand method. This uses the formula of AD in order to count GDP. 
AD = C+I+G+X-M, where C is consumption by households, I is investment, G is government spending, X is exports and M is Imports. 
3) The Income method, where GDP = sum of different incomes such as Income of Employed and self-employed, profits of firms and rent income. 

When measuring the level of national income and whether it grows or declines it is important to look at some factors such as overall economic growth, changes in living standards distribution of income and also purchasing power of money to compare with other countries.