Here’s my understanding of the situation on a more technical level. Feel free to correct me where I’m wrong.
To buy residential property, masses of people took out non-recourse debts, secured against the property they were buying. That means people could default on their loans, and the banks could only collect the property. They couldn’t “reach into” the lendees’ assets on a default. Based on most of the last 20 years, banks thought residential property was a safe investment. On a <i>nation-wide</i> scale, property rarely decreases in price too much or for too long. That’s what “diversification” was all about: having security interests in so many pieces of property in so many places, that all the risks were canceled by the rules of probability.
What the banks did not anticipate (or perhaps refused to see) was that on a <i>nation-wide</i> scale, demand for houses was far higher than it “should” have been. Blame the tax code, which creates huge incentives for buying residential property (e.g., no tax on profits from sale of a home whose value has appreciated, or the home-mortgage-interest deduction from gross income). Blame overeager economists, businessmen, lawyers and politicians, and of course <i>banks</i>, who convinced ordinary people that home-owning was the way to go. The banks didn’t realize they were doing themselves in. So, for a while, people bought more residential property than they needed (or could afford), because they had inflated expectations of gain from doing so.
Within a few years, as the economy weakened and people sobered up, they realized 1) owning residential property wasn’t yielding them the gains they’d hoped for, and as a result, 2) they couldn’t <i>afford</i> to pay off the mortgages (loans) on their property. So they did the logical thing: they tried to sell their houses. This would have worked in most eras. High supply means low prices, which means an influx of buyers from elsewhere where prices are higher. But remember, this was happening nation-wide. There were no low-supply buyers elsewhere to rush in and exploit the excess supply. What happens when people can’t pay their mortgages and can’t sell their property? They default on their loans, of course.
The disaster’s now showing pretty clearly. The banks seize the residential property from defaulting lendees. The banks had planned on that property appreciating in value, so it’d be worth a lot more today than it was 5 years ago. Instead, it’s worth <i>significantly less</i>. If these were <i>recourse</i> debts, the banks could reach into the defaulters’ bank accounts and assets to get the rest of what the defaulters owed them. But because these were non-recourse debts, the banks were stuck with property worth very little. Even worse, the banks couldn’t <i>sell</i> the property because nobody wanted it. So the banks couldn’t even liquidate the devalued property.
Meanwhile, seeing that the investment banks are in a bad situation, people start withdrawing their money very quickly. This was a logical decision, because clearly a catastrophe was looming on the horizon. But of course it exacerbated the problem: investors were now demanding their money from the banks, who couldn’t spare money to give, and couldn’t convert their assets to money because nobody wanted the assets. Remember seeing the expression on the Morgan Stanley CEO’s face? Remember how he kept shouting that the market was behaving irrationally and it threatened to pointlessly destroy investment banks? This is what he was talking about. Only, the market was <i>quite</i> rational to be withdrawing all of its investments. It’s just that doing so came at the expense of investment banks. Fortunately for that CEO, Morgan Stanley was one of the lucky two that survived.
So that’s what happened. Three investment banks ran out of money from all the withdrawals, and went bankrupt or were bought out. The other two barely scraped by, and they’ve given up their ability to take risks (see: all of the above) in exchange for the safety of government backing. In reaction to the financial crisis, we’re going to see a ton of safe behavior from everyone. What does that mean? Lots and lots of government regulation against risk-taking. Basically, one giant leap toward socialism. And frankly, I can’t blame people for wanting some safety and slow, old-fashioned progress. It’s just a shame that it had to come to this, when <i>moderate</i> risk-taking could keep our economy moving at a fairly rapid rate, without triggering a long, slow-as-molasses recovery period like this.