APR and APY are both financial terms used to express rates, but they serve different purposes and convey different information.

Annual Percentage Rate (APR) is commonly used for loans and credit cards. It represents the yearly interest rate you'll pay for borrowing money, without considering the effects of compounding. APR includes not only the interest rate but also other costs and fees associated with borrowing, providing a more accurate representation of the cost of the loan.

Annual Percentage Yield (APY), on the other hand, is typically used for savings accounts and investments. APY represents the effective annual rate of return, taking into account the effect of compounding interest. The more frequently interest compounds, the higher the APY will be compared to the nominal interest rate. APY allows you to easily compare the earning potential of different savings accounts and investment options.

In summary, APR is used to express the cost of borrowing money, while APY is used to represent the earning potential of savings and investments.