a) Different sectors of economy - There are three sectors of the economy

i) Primary sector- It includes all those production units which exploit natural resources for production. Such as Agriculture, mining, quarrying etc. 

ii) Secondary sector- It includes all those production units which transform one good in to another. Such as Industries, construction etc. 

iii)Tertiary sector- It includes all those production units which provide services. Such as banking, insurance, transport, communication etc. 

b) Injection: The amount of money which is added in the circular flow of income is called injection. Injections increase the speed of flow of income. Some examples of injections are investment (I), government expenditure (G) and export (X).

c) Withdrawal: The amount of money which is taken out from the circular flow of income is called withdrawal. Withdrawals decrease the speed of flow of income. Some examples of withdrawals are savings (S), taxes (T) and import (M).

d) Double Counting – If the value of an item is estimated more than one time in estimation of national income is called double counting. It leads to over estimation of national income.

It can be avoided by two ways-

i) By taking the value added by each enterprise 

ii) By taking the value of final product

e) Depreciation: The reduction in the value of capital assets due to normal wear and tear is called depreciation. These are the foreseen losses so cannot be insured. It is also called consumption of fixed capital.

Whenever we subtract net value from the gross value we get depreciation. Depreciation = Gross – Net