Harry Markopolos sent this letter to the US Securities & Exchange Commision in 2005, which was a follow up to one he sent in 1999. It's pretty blatant and to the point that Madoff was a scammer. His letter was basically ignored.
One of the fund managers who invested his clients money ($1.4bn IIRC) with Madoff commited suicide when Madoff came clean just before Christmas that he was running a scam.
Madoff had been running a Ponzi scheme for the last 14 years. He took billions of dollars from new investors, and used that money to pay the previous investors their "profits" which they thought he had been earning them from investing for them as a fund manager. It's a bit like a pyramid scheme, but more of a vertical line than a pyramid.
It's affected banks all over the world (RBS, who I've invested in, have lost $400m I think), charities, general rich people like Hollywood people, and other investment funds whose fund managers thought they were privileged enough to be given the honour of having Bernard Madoff manage some of their client's money for them.