APY is the rate of return you will have receive on your investment in a year.
The formula to calculate APY is (1 + r/n)^n - 1. The "r" is the interest rate in decimal form (3.45 percent would be written as 0.0345), "n" is the number of compounding periods per year and "^" is to the power of. If the interest rate is compounded quarterly, then "n" would be 4. Using these numbers in this formula would equate to a 3.49 percent APY.
Banks and other financial institutions use APY to advertise interest-bearing accounts. You will see APY being quoted on savings accounts, interest-bearing checking accounts, money market accounts and certificates of deposit.
Compounding is simply making earnings of your earnings or previously accrued interest. The more the interest is compounded, the higher the APY. APY can be compounded yearly, quarterly, monthly or daily.
You can use the APY formula to compare how different interest rates with different compounding frequencies affect your total interest. For example, a lower interest rate that is compounded more frequently may give you a higher APY than a higher interest rate compounded less frequently.