Simply put, a mortgage is a loan secured by real property and paid in installments over a set period of time.
The mortgage secures your promise that the money borrowed for your home will be repaid.
According to Wikipedia:
A mortgage loan is a loan secured by real property through the use of a document which evidences the existence of the loan and the encumbrance of that reality through the granting of a mortgage which secures the loan. However, the word mortgage alone, in everyday usage, is most often used to mean mortgage loan.