A short sale is a pre-foreclosure property where the owner expects to sell the property for less than what is owed on the mortgage.
Usually these sellers are highly motivated because the bank will only allow a limited amount of time for the owner/borrower to sell the property before they schedule it for auction.
Unfortunately, short sales are extremely long transactions (average of 4 - 6 months to close) and have a high rate of transaction failure. That being said, you can often find below market deals.