An adjustable rate mortgage is a mortgage product which has an interest rate that changes periodically. As opposed to a fixed rate mortgage which has the same interest rate for the life of the loan, ARMs typically have a period where rates are fixed (typically between 3 and 10 years), followed by a variable rate structure which is tied to a market index such as libor plus a margin. Payments will fluctuate as interest rates move up or down. In general, interest rates on ARMs will be lower than interest rates on fixed products in order to compensate the borrower for the added risk of having a variable payment at some point in the future.
Borrowers gravitate to ARMs for a variety of reasons:
Part 2 - ARM Basics
Part 3 - Types of ARMs