Interest
Interest is the money paid by a borrower to the lender for the use of money borrowed.
In other words;
The additional money paid for using other's money is called interest.
Some key things about Interest.

Principal or sum :
Principal or sum is the money,that borrowed or lent out for a certain period. 

Time :
It is the duration of period for which the money is borrowed. 

Amount :
The sum of Principal and Interest is called Amount. 
Amount = Principal + Interest 

Rate of Interest :
The Interest rate of the principal for a certain period. 
Example:
Let, 9% per Annum. 
It means, the interest on principal of Rs. 100 for one year is Rs. 9. 