The presumption is that you will go through the normal means of procurring your loan. That is, you cannot just assume the loan your parents have. From a financing angle, this will not be any different then if you bought the home from a non family member. You will need to qualify for financing on your own, and you will need to come up with the downpayment. The escrow will commence and once your lender funds the loan, those proceeds will go to pay off your parents loan and the balance will go to your parents.
If your parents decide to give you a break on the price and sell you the home lower then market value that should even be okay as long as they appraise. However, the tax assessor will base your property tax on fair market value. You will also need to make sure that you purchase the correct amount of homeowners insurance should you get the home below market value.
