These loans generally begin with an interest rate that is lower than a comparable fixed rate mortgage. The interest rate changes at specified intervals (for example, every year) depending on changing market conditions; if interest rates go up, your monthly mortgage payment will go up. However, if the index goes down, your mortgage payment will drop also.
There are also mortgages that combine aspects of fixed and adjustable rate mortgages - starting at a low fixed rate for seven to ten years, for example, then adjusting to market conditions. Ask your mortgage professional about these and other special kinds of mortgages that fit your specific financial situation.