Mortgage Interest Rate vs Mortgage APR

What is the difference between the mortgage interest rate and the mortgage APR?
You'll see an interest rate and an Annual Percentage Rate (APR) for each mortgage loan you see advertised. The easy answer to "why" is that federal law requires the lender to tell you both.
The APR is a tool for comparing different loans, which will include different interest rates but also different points and other terms. The APR is designed to represent the "true cost of a loan" to the borrower, expressed in the form of a yearly rate. This way, lenders can't "hide" fees and upfront costs behind low advertised rates.
While it's designed to make it easier to compare loans, it's sometimes confusing because the APR includes some, but not all, of the various fees and insurance premiums that accompany a mortgage. And since the federal law that requires lenders to disclose the APR does not clearly define what goes into the calculation, APRs can vary from lender to lender and loan to loan.
The APR on a loan tied to a market index, like a 5/1 ARM, assumes the market index will never change. But ARMs were invented because the market index changes and makes fixed rate loans cheaper or more expensive to make -- that's why they're variable rate in the first placed!
So, APRs are at best inexact. The lesson is, that APR can be a guide, but you need a mortgage professional to help you find the truly best loan for you.
Note when you're browsing for loan terms that the APR will not tell you about balloon payments or prepayment penalties, or how long your rate is locked. Also, you'll see that APRs on 15-year loans will carry a higher relative rate due to the fact that points are amortized over a shorter period of time.