what is principal and interest 
your principal is the money that you originally agreed to pay back interest is the cost of borrowing the principal for example if the interest rate on a 100 000 mortgage is 6 the combined principal and interest monthly payment on a 30 year mortgage would be about 599 55 500 interest 99 55 principal for the first payment over time the amount of the payment stays the same but the proportions change so that eventually more of the payment goes toward the principal and less toward interest 