With a repayment mortgage you make monthly repayments for an agreed period (this is known as "the term") until you've paid back the loan and the interest.
With an interest only mortgage you make monthly repayments for an agreed period but this will only cover the interest on your loan. You'll normally also have to pay into another savings or investment plan that'll hopefully pay off the loan at the end of the term.
You will need to choose one or maybe a combination of savings and investment plans to cover the capital part of your mortgage. There are many around but to name a few, you can take out an ISA, an endowment or even use tax free cash from your pension plan.
It is crucial that you regularly review your savings or investment plan to ensure that it is performing as expected and will pay out enough money to repay the mortgage at the end of the term.
If your savings plan doesn't do so well, you will be left with a shortfall.
In most cases, you may be tied into a particular mortgage product for an agreed period. If you try to pay off your mortgage during this period then, in some cases, a penalty called a "redemption charge" may apply.
At the end of the agreed period you will then have the choice of staying with the same lender or switching to an alternative lender (this is known as "remortgaging"). If you stay with the same lender, then you will normally be automatically put onto their standard variable rate which may be higher or lower than, for instance, a fixed rate you have been paying for years. So watch out for this and make sure you can cope with any changes to your monthly repayments.