A scheme in which the victim is offered a commission on a large sum of supposed inaccessible money if the victim will pay a (nonexistent) tax or fee that stands in the way of the collection of that money.
Such scams are usually done using stolen or remarked official letterhead, to lend credibility. Most of these scams originate from Nigeria, where tracing a phone call (sometimes used to initiate these scams) is nearly impossible due to the ad hoc nature of their phone network.
Also known as a 419, after the section of Nigerian law that covers this type of fraud, or simply AFF.