Most financial institutions offer different tiers of savings accounts to customers. The highest interest rates usually go to customers with the largest balances. This is because the more capital a financial institution has from customer balances, the more it can invest and claim as assets. Interest rates get even higher for customers who are willing to put money into savings accounts that are a little more access-restrictive, such as a Money Market account CD's. High interest rates are often a feature of high-risk investment savings accounts too.
Interest can work upon interest when compound interest comes into play. This is what happens when the interest you earn becomes a portion of your balance and therefore starts to collect interest. In other words, you're earning interest off the interest the financial institution has paid you. Compound interest is a way to get the most out of interest-bearing savings accounts, and it's one of the main reasons why a higher interest rate can be such an effective means of making extra money if you have a substantial balance in a savings account.