In legal terms, a mortgage is "the pledging of property to a creditor as security for the payment of a debt" [source: YourDictionary.com]. In plain English, a mortgage is a loan. For many people, it's the biggest loan they will ever borrow. With a regular loan, there's no explicit collateral. The lender looks at your credit history, your income and your savings, and determines if you're a good risk. With a mortgage, the collateral for the loan is the house itself. If you don't pay back the loan (along with all of the fees and interest that are included with it), then the lender can take your house.
Banks are the traditional mortgage lender. You can either apply for a mortgage at the bank you use for your checking and savings accounts, or you can shop around to other banks for the best interest rates and terms. If you don't have the time to shop around yourself, you can work with a mortgage broker, who sifts though different lenders to negotiate the best deal for you. Banks aren't the only source of mortgages, though: Credit unions, some pension funds and various government agencies also offer mortgages.
Like other loans, mortgages carry an interest rate, either fixed or adjustable, and a length or "term" of the loan, anywhere from five to 30 years. Unlike most other loans, mortgages carry a lot of associated costs and fees. Some of those fees only happen once, such as closing costs, while others are tacked onto the mortgage payment every month.