deakymom - interest only means just that, you've borrowed say £200k, at no point are you paying back any of that debt, just paying the interest on the debt. At the end of the term, you still owe £200k, so interest only mortgages are only a good idea for a short period of time if you are going to be financially stretched for a limited period (say, on mat leave but then going back to work or dropping to one wage if the other person is going back to study for a set period etc), or you have some other way of paying off the £200k, like a different investment plan that would give you £200k at the end of the term, or a trust fund that would pay out at some point etc.
Unfortunately, a lot of people's 'plan' was that house prices would rise, they'd buy a house for £200k, pay the interest only, then in 5-10 years time, they'd have a house worth £300k, remortgage then with £100k equity so get a better rate, and start paying it back. Others just haven't thought at any point that they would pay back the debt, just that they'd own it for so long that it would be such a tiny part of the total value it wouldn't matter, or their financial situation would improve and they'd start being able to pay it back.
being in negative equity means that the "plan B" of "if it all goes wrong, we can sell up and rent" doesn't work because they still owe £200k but now own a house worth only £180k, so the bank would still want their £20k back, and at the point you sell the asset the loan is secured on, you have to pay it back.
This 'traps' you if you need to move areas, because unless you can sell your first house and afford to pay the bank back the difference, then you can't move as they will only give you a mortgage for the second house that's to it's value, if the second house is worth £200k they will only loan you the money to buy that house, you still will have to use the money you get from the sale of the first house to pay back the debt on the first house, and find the cash yourself for the shortfall. You can't carry negative equity to the second house, as that's effectively a new loan.
Hope that makes sense!