An e-mail is sent to the victim’s inbox, supposedly from a hitman who has been hired by a “close friend” of the recipient to kill him/her, but will call off the hit in exchange for a large sum of money. This is usually backed up with a warning not to contact the local police or FBI, or the “hitman” will be forced to go through with the plan.  Technically an extortion scam, it is however perpetrated by the same criminal groups that employ other advance fee fraud schemes, and the “payoff” could be construed to be the victim’s life which, as in most advance fee fraud, the scammer usually does not have the means to take. In a common twist, after initial payments the hitman will offer to instead kill the person who originally ordered the hit, or a second hitman will contact the victim and offer to kill the first. The victim may agree and pay for the hit, thinking the problem will go away, but the scammer will often use these plot twists to extort even more money by playing both hitmen, or the hitman and the unseen contractor, against the victim.
Source : wikipedia