Federal law requires that A.P.R. be disclosed along side the actual interest rate as a means to help borrowers make a more informed decision on their mortgage. The interest rate (aka the nominal rate), is the percentage rate that the lender applies to your loan balance. The A.P.R. (aka the annual percentage rate) is the combination of your interest rate plus other charges and fees. It is designed to represent the "true cost of the loan, expressed in the form of a yearly rate so lenders can't hide fees and upfront costs behind low advertised rates. In a nutshell, the A.P.R. is supposed to help you compare different loans, which will include different interest rates and also different points and other terms.
While it's designed to make it easier to compare loans, it's sometimes confusing because the A.P.R. 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 A.P.R. does not clearly define what goes into the calculation, A.P.R.s can vary from lender to lender and loan to loan.