This one involves an item you might have listed for sale such as a car, truck, or some other expensive item. The scammer finds your ad and sends you an email offering to pay much more than your asking price.
The reason for overpayment is supposedly related to the international fees to ship the car overseas. In return, you are to send him the car and the cash for the difference. The money order you receive looks real so you deposit it into your account. In a couple of days (or the time it takes to clear) your bank informs you the money order was a fake and demands you pay that amount back immediately.
In most documented versions of this money order scam, the money order was indeed an authentic document, but it was never authorized by the bank it was stolen from. In the case of cashier's checks, it is usually a convincing forgery. You have now lost the car, the cash you sent with the car, and you owe a hefty sum of money to your bank to cover for the bad money order or the fake cashier's check.