Hybrid ARM mortgages, also called fixed-period ARMs, combine features of both fixed-rate and adjustable-rate mortgages. A hybrid loan starts out with an interest rate that is fixed for a period of years (usually 3, 5, 7 or 10). Then, the loan converts to an ARM for a set number of years. An example would be a 30-year hybrid with a fixed rate for seven years and an adjustable rate for 23 years.
The beauty of a fixed-period ARM is that the initial interest rate for the fixed period of the loan is lower than the rate would be on a mortgage that's fixed for 30 years, sometimes significantly. Hence you can enjoy a lower rate while having period of stability for your payments. A typical one-year ARM on the other hand, goes to a new rate every year, starting 12 months after the loan is taken out. So while the starting rate on ARMs is considerably lower than on a standard mortgage, they carry the risk of future hikes.