What is the effect of paying extra principal on mortgage?

Long answer - I explained *why* below the simple answer in case you needed it.  :)
Paying extra towards your principle on a loan pays it off faster.  Extra payments directly paid to principle can make a huge difference in the life of the loan.  Even just a few dollars extra towards principle a month can make a considerable difference, and while it may not seem like much extra to you, that little bit extra each month can literally take years off the loan (but it's super important to make sure they put the funds where you want it - principle and not interest, etc.) While your monthly payment is the same each month, the way it is allocated each month is not-and it changes monthly.  Lenders divide monthly payments so that most of the payment is paying off the interest at the beginning of the loan and very little of the monthly amount is going toward principle.  This is done because many people will eventually pay off their loans early and the lender makes their money off the interest being paid to them.  An example would be a monthly payment of $750.00.  The beginning of the loan it could be divided with $700 going to interest and just $50 going to the principle...and each month it changes little by little  The next month it may be $698 to interest and $52 to principle.  Towards the end of the loan it will be exact opposite with most of the payment allocated to principle and very little to interest, because the interest has been paid up front.