In a Ponzi scheme, you have an organization that takes peoples money, does some complicated things with it that are difficult to understand, pays internal people handsomely, pays back early "investors" handsomely, then suddenly collapses and all the investment the later investors thought they had is gone. This happened in 2007 before the governments of the world contributed in $3,000,000,000,000 keep the scheme going.
The difference between the Wall St. Ponzi scheme and your average couple-million Ponzi scheme is three-fold: the organizers are still running it instead of being in prison, it is a million times bigger, and scheme insiders are running the Federal Reserve and the U.S. Treasury.
This is be made out to be a natural consequence of the complicated modern financial world, but was it unavoidable? The $3T bailout works out to about $10,000 for every person in the U.S.