When you've selected your loan, you'll decide whether you want a fixed or an adjustable rate. Your choice defines the interest you'll be charged.The interest rate on a fixed-rate loan never varies. If you're established in your career, or have an expanding family and are ready to settle down, a 15- or 30-year fixed-rate loan might be your best bet, because you'll always know what your monthly mortgage payment will be. It's worth mentioning, though, that if other fees are trundled into your monthly mortgage payment there may be certain fluctuation occasionally.Adjustable-rate mortgages, or ARMs, have interest rates that reset at certain intervals. They normally begin with lower interest rates than fixed-rate loans; sometimes these are referred to as teaser rates. After the preliminary term ends, the interest rate -- and your monthly payment -- increases or decreases annually based on an index, plus a margin. This type of rate most often appeals to more mobile buyers who plan to stay in their homes for just a few years or refinance when the teaser rate is about to end. Paying a lower interest rate in those initial years could save hundreds of dollars each month that could fund other investments.Review the difference between the two rates to determine which suits your needs best.