If a house is put in foreclosure, what is the legal obligation of the defaulter to the bank if the home is sold for less than the defaulted loan?

I am sorry for the situation.  If the hosue is sold for less than the mortgage amount then there is what is known as a "deficiency" and the lender may be able to come after you for the funds.  They take what is known as a "deficiency judgement" against you and then they can collect by the means allowed under your state law.  Please, please go and speak with some one about options even at this [possible late date.  You can do a deed in leiu of foreclosure or a short sale but asking that the lender waive the deficiency.  Good luck.