Nigerian scams are a form of advance-fee scam and one of the most common types of confidence frauds.
The scam typically involves promising the victim a significant share of a large sum of money, in return for a small up-front payment, which the fraudster requires in order to obtain the large sum. If a victim makes the payment, the fraudster either invents a series of further fees for the victim or simply disappears.
To captivate the victim’s interest, an extraordinary story must be told. The victim usually receives an unsolicited e-mail or letter stating that millions of U.S. dollars need to be removed from Nigeria (or some other country, depending on the current geopolitical situation) and you have been selected by government, banking officials or surviving relatives to assist. The message usually is unspecific to the recipient (i.e., does not contain valid data about the recipient/victim) and states that, "You will be amply rewarded for your assistance by retaining a percentage of the funds transferred; however, in order to facilitate the procedure your financial assistance is required up front."
Once the victim answers the letter, usually including some of his/her personal data, he/she may receive another letter along with official-looking documents. The accompanying letter is more specific and informs the victim that he/she will have to provide money for the funds to be released. Naturally, once the victim sends money requested, the scam is complete – there is no percentage of the funds being transferred. The victim has lost his/her initial "investment."
In other cases, payment of taxes, bribes to government officials, and legal fees are often described in great detail with the promise that all expenses will be reimbursed as soon as the funds are spirited out of Nigeria or the selected country. Once the victim realizes it was a scam and stops sending money, the scammers have been known to use the personal information or checks that they received to impersonate the victim, draining bank accounts and credit card balances. Some victims have been lured to Nigeria, where they have been imprisoned against their will along with losing large sums of money.
(Based on an article from www.fbi.gov/scams-and-safety)