Mortgages are not a tax, though they are considered liabilities in accounting. A liability being anything that has to be paid out and so would be a negative entry in a balance sheet.
They tend to be huge, because of the assets being purchased. Though, this depends greatly on the housing market in the particular location. This varies greatly in the U.S.
No, it is not a security deposit on a loan. It is a loan. The asset being bought is typically the collateral on the loan. If you default on the loan, the lender takes possession of the house and typically boots you out so it can sell it in order to get it off its balance sheet.
Yes, most Americans buy their homes through loans. I would expect this to be typical in many countries, due to the relatively high value of these assets. It would take quite some time to save up the money in order to pay cash. The basic idea is to trade-off that time and make the purchase accessible to more people at an earlier time. It involves some risk on the part of the buyer and the lender.
The total value of the mortgage is the price at which the asset is bought plus the interest payments over the life of the loan, though the value to the lender is the sum of the interest payments. The typical terms are 30-year, fixed-interest. Though there are adjustable rate mortgages (ARM) and shorter term ones available. These are typically avoided due to the higher interest rate on the shorter terms and the possibility of the ARM adjusting to prohibitively higher interest rates. That said, they have their uses and judicious employment of them can be a benefit.