The victim receives a call from someone who excitedly announces that the victim has won the lottery of another country (usually Canada, Australia or Mexico), even though the victim never entered.
However, the victim must prepay taxes or fees before the substantial winnings can be sent. The victim must act quickly or lose "their" winnings.
Typically they are told to wire the money via Western Union. When this is done, the victims receive nothing or are told that they must send more and more money.