Y=C+I+G+(X-M)
With Y representing National Income, C is Consumption Expenditure, I if Investment Expenditure, G is Government Spending, X is Exports and M is Imports.
So if we remember that: National Input = National Output = National Expenditure, then a good way to have a generic measure of them all is to refer to Aggregate Demand such that:
AD=C+I+G+(X-M)
By using Aggregate Demand we are able to observe changes in its components and see the effects on the national economy
A movement along the curve could be caused by an increase in general price level meaning there would be a lower real national output.
A shift of the curve (the whole line moving wither right or left) could be caused by the total quantity demanded of goods and services at a given price level. Also if imports increase, aggregate demand would decrease.
