When purchasing a home, a 20% down payment is typically the standard. PMI (Private Mortgage Insurance) protects the lender if a borrower doesn't pay on the loan and the market price of the house is less than the balance of the loan.
Since the $40-$50 a month per $100,000 borrowed is compiled into the mortgage payment and often isn't even tax deductible, PMI can be costly to a borrower.