A loan specifically for the purchase of a piece of real estate. For most people, a mortgage is by far the largest loan they will ever have to ask for. Because of this, and because of the unique nature of real estate as property, mortgages have a set of laws and regulations which are totally unique in the financial world.
Most mortgages will allow you to roll property taxes, assessment fees, and other costs often associated with buying real estate together with the actual purchase price of the property. This is a simple way to simplify the costs of making the purchase, and is almost always a good idea. The cost of those fees and taxes is nominal compared to the cost of the property itself, anyways.
Contrary to what yaqub0r asserts, however, a mortgage is not a scam. Like any other loan, you eventually end up paying far more money than you borrowed. However, real estate is one of the few major purchases you can make that is almost guaranteed to appreciate in value, provided you take care of your purchase. A well-handled mortgage can actually make you money if you hold on to the property for long enough, although in the financial world "long enough" usually means several decades. In addition, purchasing a home will almost always cost less over time than renting property, provided you stay in one place for a number of years.