They'll send you a fake email indicating that they've sent you a payment of some amount and they will overpay by a few thousand dollars. Really no payment had occurred. Then they'll ask you to send back the difference in payment.
There's a great article on beating the scammers:
How to Beat Nigerian Scams - WSJ.com
Essentially, responding to scammers and toying with them will reduce their efficiency on reaching others as they have to spend time following up. The real solution here is to use automated bots to do this.