Pyramid selling: You give me one pound, so that I can "invest" it, and I promise to pay you back two pounds. I persuade two more people to give me a pound each, and then I give you the two pounds which I got from them. I persuade four more people to invest a pound, and I give those four pounds to the two people who gave me the pound each. Then I persuade eight more people.... etcetera. There are soon not enough people left, so the last lot of investers lose their money.
Went to a London recruitment seminar for AMWAY about ten years ago. Lots and lots of chat about how easy it is to make a stack of money....then right at the end they tell you it's about having a garage full of washing powder and pan scourers and then selling them on to your family and friends. AMWAY is actually an acronym of Selling Stuff You Can Buy In Any Supermarket Door To Door......