modern finance

An advertisement is placed promising extraordinary returns on an investment – for example 20% for a 30 day contract. The precise mechanism for this incredible return can be attributed to anything that sounds good but is not specific: "global currency arbitrage", "hedge futures trading", "high-yield investment programs", "Offshore investment", or something similar.

With no proven track record for the investors, only a few investors are tempted, usually for smaller sums. Sure enough, 30 days later the investor receives the original capital plus the 20% return. At this point, the investor will have more incentive to put in additional money and, as word begins to spread, other investors grab the "opportunity" to participate. More and more people invest, and see their investments return the promised large returns.

The reality of the scheme is that the "return" to the initial investors is being paid out of the new, incoming investment money, not out of profits. No "global currency arbitrage", "hedge futures trading" or "high yield investment program" is actually taking place. Instead, when investor D puts in money, that money becomes available to pay out "profits" to investors A, B, and C. When investors X, Y, and Z put in money, that money is available to pay "profits" to investors A through W.
One reason that the scheme initially works so well is that early investors – those who actually got paid the large returns – quite commonly reinvest (keep) their money in the scheme (it does, after all, pay out much better than any alternative investment). Thus those running the scheme do not actually have to pay out very much (net) – they simply have to send statements to investors that show how much the investors have earned by keeping the money in what looks like a great place to get a high return. They also try to minimize withdrawals by offering new plans to investors, often where money is frozen for a longer period of time, for example 50% return per month for one year. They then get new cash flows as investors are told they could not transfer money from the first plan to the second.

The catch is that at some point one of three things will happen:

the promoters will vanish, taking all the investment money (less payouts) with them;

the scheme will collapse of its own weight, as investment slows and the promoters start having problems paying out the promised returns (and when they start having problems, the word spreads and more people start asking for their money, similar to a bank run);

the scheme is exposed, because when legal authorities begin examining accounting records of the so-called enterprise they find that many of the "assets" that should exist do not.

http://en.wikipedia.org/wiki/Ponzi_scheme