Interest-only is an option that can be attached to any type of mortgage.
For example, a 30-year fixed rate mortgage of $100,000 at 6% has a monthly payment of $599.56. This is the fully amortizing payment -- the payment which, if maintained over the full term of the loan, will just pay it off.
In month 1, that payment divides into $500 of interest and $99.56 of principal. In month 2, the payment remains at $599.56 but the breakdown is $499.50 and $100.06. Each month, the interest portion declines and the principal portion rises. After 5 years the balance is $93,054. That is how mortgages amortize.
Now lets attach an interest-only option to this mortgage, available, say, for the first 5 years. That means that the borrower need pay only $500 a month during the first 5 years. There is no payment to principal.
If the borrower exercises the option, therefore, the balance after 5 years is $100,000. There is no amortization. Beginning year 6, the borrower must begin paying $644.31. That is the fully amortizing payment for a 6% loan of $100,000 for 25 years.