With a repayment mortgage you pay off both the interest and capital with each monthly payment. This means at the end of the term your mortgage will be repaid.
With an interest only mortgage you only pay the interest each month and the amount of capital owed will not reduce.
This means you need to have suitable plans in place to pay off the mortgage at the end of the term. You could use sale of the mortgaged property, cash, stocks and shares, an endowment policy or pension.