Interest is the charge for the privilege of borrowing money, typically expressed as annual percentage rate.
There are two main types of interest that can be applied to loans: simple and compound. Simple interest is a set rate on the principle originally lent to the borrower that the borrower has to pay for the ability to use the money. Compound interest is interest on both the principle and the compounding interest paid on that loan. The latter of the two types of interest is the most common.