A repayment mortgage costs more, but guarantees ownership of your home at the end of the term. Monthly repayments go towards clearing the interest and paying back your initial borrowing.
In this way, if you choose to move house partway through the term, you will only have to pay off the remainder of what you owe. This may mean you can borrow less against your new property, or take a new mortgage out over a shorter term.
Some people choose to take out an interest-only mortgage to begin with, then switch to a repayment mortgage when they have more spare cash. If you are experiencing temporary financial problems or expect a wage increase in the future, this might be a good solution.