If a deposit account of $100 earns an interest rate of 10% a year, then at the end of the year the account will contain $110. If all of that money is left in the account, then the 10% interest will be paid on the $110, so at the end of the second year $11 of interest will be added, making $121 in all. This is known as compound interest. By contrast, simple interest pays the 10% only on the original sum in the account.