Here's a pretty good NYT article on the LTCM bailout in 1998
http://marshallinside.usc.edu/dietrich/NYTimesLTCM2.htmBasically, a hedge fund takes money from 'expert' investors - banks, trusts, other sorts of funds and rich folk. They build up a staff of mathematicians, economists and build super-duper computer models of economies and markets. They analyze long strings and networks of investments that cross markets. Their models try to account for unusual inter-market interactions which don't have the usual market efficiencies because they are not part of any normal business deals.
They make huge complex multi-part deals that involve scores of transactions as part of one investment. Then they leverage as many pieces as possible - basically borrowing money cheaply and incorporating it into the deal. So one deal might involve a billion dollars plus another 5 billion in leverage, and then make or lose $100 Million in a few weeks. If they can 'win' on 12 out of 20 deals in a year, then they have made $400 million on a $1Billion investment, and earn a 40% return. They might have 50 such deals going at any given time - $50 Billion leveraged to $300 Billion.
But when something goes wrong, it can affect a dozen of their 50 deals and in turn jeopardize all 50 of the deals and BOOM you have $300 Billion worth of (to anyone else) nonsense trades all going bad at the same time in markets all over. Throws all the markets off kilter and messes up the big deals of other hedge funds and pretty soon you have a multi-trillion dollar mess and the companies who provided the leverage are caught up in it.
So basically, a bunch of rich folk and money businesses - who should know the risks - will have made a bad bet and should loose their shirts. But they have entangled so many other companies and jeopardized entire markets, that they become 'too big to fail' and have to be bailed out. They get their 40% returns for years, but when the inevitable 300% loss comes along to balance things out, they are protected - must be protected because they've risked entire economies and markets.
Thing is that they're investments are so esoteric and proprietary, that they should not be able to get any leverage for their trades at all. No bank is really able to judge the risk involved and hence should not be providing credit for the leveraging.