Too many people die in ghastly car accidents or air crashes in Nigeria. Some accidents are ghostly, rather than ghastly, because the sender can't spell. Some of these people get cancer and die, some get run over and die but, fortunately, they all die rich and have no relatives. This is where you come in and stand every chance of not having to work again.
The email, usually from a bank employee or lawyer, asks you to provide details (which you wouldn't give to your best friend) and you are promised a commission ranging from 10% to 30% for getting the money transferred out of Nigeria (or sometimes it might be a different, but equally wealthy, African country).
Other versions of the scam play on your charity, loneliness, or naiveté (you can't win a lottery you didn't enter). Orphan, cancer patient, dead bank customer, phony job offer, overpayment with a cashier's check... same scam.