What is the difference between a repayment mortgage and an interest only mortgage?

A repayment mortgage is guaranteed to pay off your mortgage by the end of the term as long as all payments have been made. With an interest only mortgage your monthly payments only pay the interest that is due so at the end of the term you still owe the same amount that you originally borrowed and would need to either sell your property to repay the mortgage or find the money to repay it from another source by that tim.