Explain clearly the various approaches used in measuring national income?

Answer 1

National income can be measured by:

The two most common methods:
GDP
GNP

Less used methods:
GNI
NNI
NNP

Each can be measured by income method (counting how much money everybody makes) and the expenditure method (counting how much everyone spends and saves).

I could write a long article, but frankly someone at Wikipedia did a darn fine job:
http://en.wikipedia.org/wiki/Gross_national_product