In a traditional mortgage, a borrower pays a fully-amortized monthly payment. This means that they are paying the exact amount necessary in order to pay their mortgage off in full by the end of their term. Interest-only mortgage loans differ in that they do not require fully-amortized payments at the beginning of the mortgage term. This article explains how interest-only mortgage loans work:

Interest-Only Payments

For a period of time established by your lender -- usually a few years -- interest-only mortgages only require that a borrower makes monthly payments on the interest accrued on their loan. This means that the borrower is not required to pay any amount on the principal. This makes for monthly payments that are considerably lower than fully-amortized payments.

Conversion to a Traditional Mortgage

After the term of interest-only payments, the loan converts to a traditional mortgage. This means that you will be responsible for fully-amortized payments for the remainder of the mortgage