A fixed-rate mortgage is a type of mortgage where the interest rate stays the same for the entire term of the loan, usually 15 or 30 years. An adjustable-rate mortgage (ARM), on the other hand, has an interest rate that can change over time based on market conditions. ARMs typically have a lower initial interest rate than fixed-rate mortgages, but the rate can increase over time, which can make it harder to budget for monthly mortgage payments.