An interest-only loan is a loan in which for a set term the borrower pays only the interest on the Principal balance, with the principal balance unchanged. At the end of the interest only term the borrower may enter a interest-only mortgage, pay the principal, or (with some lenders) convert the loan to a principal and interest payment (or Amortized) loan at his/her option.
In the United States, a five or ten year interest-only period is typical. After this time, the principal balance is amortized for the remaining term. In other words, if a borrower had a thirty year mortgage and the first ten years were interest only, at the end of the first ten years, the principal balance would be amortized for the remaining period of twenty years. The practical result is that the early repayments (in the interest-only period) are substantially lower than the later repayments. This enables a borrower who expects to increase their salary substantially over the course of the loan to borrow more than they would have otherwise been able to afford. Interest only loans were popular in the 1920s. Due to the economic downturn and lack of work for the average person, there were many foreclosures during the Great Depression of the 1930s.
During the interest-only years of the mortgage, one is essentially renting the house since none of the principal loan decreases. The two great disadvantages are that in many states one has to pay property tax and purchase mandatory property insurance.