A short sale is a sale of real estate in which the net proceeds from selling the property will fall short of the debts secured by liens against the property. In this case, if all lien holders agree to accept less than the amount owed on the debt, a sale of the property can be accomplished.
Buying short sales can be a lengthy process, as all parties in the transaction need to negotiate and agree to a sale price that is lower than the debt obligations. These parties usually include the bank(s) holding the mortgage (and the lien), the borrower/owner of the property, the buyer making the offer for the property, and the bank (if any) issuing the new mortgage to the buyer.
Often you will see the term "Unapproved" or "Approved" short sale, to indicate if the lien holder bank and seller have settled or not on a minimum sale price that doesn't need to be negotiated further, and can be accepted automatically if a suitable buyer wants to pay the pre-agreed price.