Amortization

Amortization means that your monthly payment on your mortgage pays off both part of the interest accumulated on the loan and part of the principal balance of the loan. In the beginning,almost all of your monthly payment goes toward paying the interest accumulated on your loan. A little bit of it goes toward the principal. As less and less principal remains on the loan, less and less interest accumulates. Therefore, more and more of your monthly payment goes to principal payments. Finally, the last monthly payment pays off the remaining principal, and the debt is entirely paid.
Some mortgages involve "negative amortization". This means that the monthly payment is less than the amount of interest accumulaing each month on the loan. If you think about it, such a loan would last forever, because the borrower's monthly payment never actually goes toward the principal borrowed.