A mortgage is simply a loan using a property as security against default. A bank or another investment group gives you the money for the property with the promise that the funds will be repaid with interest. If you (the borrower) cannot make the repayments, you default and the bank gets possession of your property.
Some very useful terms that you will need to know when talking about mortgages are:

Borrower: Person taking the loan.

Lender: Institute or person giving the loan. Usually a bank.

Rate: The interest rate the lender charges.

Open vs. closed rate:
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Variable vs. fixed rate:
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Loan-to-value: How much of the property is mortgaged versus the property value.

Amortization period: The total lifetime of the mortgage. The amount of years until the loan is paid off and the property is fully owned by the borrower.

Term: How long you are locked in with the same plan with your lender

Insured/High-ratio Mortgage: Currently new mortgages in Canada that are over 80% loan-to-value must be insured by the borrower to protect the lender should the borrower default and the market go down. This means the borrow must pay a premium.

Prime: This is a government set rate that is used as an index for lenders to set their rates.

Credit: The single most important determinant in getting a mortgage is your credit; how much debt do you have, and how able are you to pay those debts.