The banks saw an opportunity to make even more money by lending to people who were highly unlikely to be able to repay the loans. This market went belly-up and the people who had taken out the loans were even worse off because now they had lost their homes. The banks were left with massive debts. They asked and are still expecting governments to give them taxpayer money to cover these debts.
The losers are the people who have lost their homes and we the taxpayers.
The winners are the bankers because in spite of having caused the problem they are still being paid annual fortunes. The other winners are the politicians and bureaucrats who failed to regulate the banks, even encouraged them to behave rashly and none of whom have lost their jobs as a result of this stupidity and are using our money to sort out the mess.
I also like Michael Rosen's take on this mess http://michaelrosenblog.blogspot.co.uk/2012/06/bailouts-and-bullshit.html