APR stands for Annual Percentage Rate. It represents the total cost of borrowing money over a year, expressed as a percentage. Unlike a simple interest rate, APR includes not only the interest but also any additional fees or charges associated with the loan. It's important because it gives you a more complete picture of a loan's cost, allowing for better comparison between different lenders.

An interest rate is simply the cost of borrowing money, usually expressed as a percentage of the principal. APR, on the other hand, includes the interest rate PLUS any other fees (like origination fees, closing costs, or discount points) that you pay to get the loan. This means the APR is almost always higher than the stated interest rate.

This calculator provides a good approximation for fixed-rate loans with a clear principal, total interest, and total fees over a set term. For more complex loans, such as those with variable rates, different compounding periods, or specific payment schedules, the exact APR calculation can be more involved and may require specialized financial software or consulting with your lender.