These loans generally begin with an interest rate that is one to two percent below a comparable fixed rate mortgage. However, the interest rate changes at specified intervals depending on changing market conditions. If interest rates go down or up, your monthly mortgage payment will follow suit. (ARM)
Note: There are also mortgages that combine aspects of fixed and adjustable rate mortgages - starting at a low fixed-rate for five, seven or ten years, for example, then adjusting to market conditions.