Robert Stewart gives a very coherent explanation of the financial crisis at Mises.org:

In summary, the essence of the subprime crisis is that money was lent (often through the agency of questionable mortgage brokers) at very low interest rates (courtesy of the Fed) to hundreds of thousands of people (all they needed was a credit score and a pulse) who could not afford to pay it back; and it was backed by collateral (a house) that was not properly valued.[7] Such assets, accurately described as “liar loans,” were then packaged into opaque securities, known as structured-investment vehicles (sponsored but not guaranteed by a respected and well-known name), which very few people understood. They were sold on to pension funds, banks, and others whose gullible investment managers also did not understand them and failed to carry out the rigorous analysis that their clients had a right to expect.[8]

Government encouraged all of this by supporting affordable housing (which was politically correct) and accusing banks of redlining (failing to lend to poor and black people in the same proportion as they lent to the rich and white). When the borrower, already maxed out on his credit cards, predictably failed to make payments, the scale of the problems eventually became apparent to somnolent regulators and financial institutions. Confidence and trust evaporated, because no one knew which institutions held suspect securities, how much the losses were, and who was ultimately safe. A financial system built on debt and excessive leverage was a financial system built on sand.

It’s a very plain english explanation of exactly what happened and I recommend it, but really it is all summed up in this one line:

It is difficult not to recall the words of Herbert Spencer: “The ultimate result of shielding man from the effects of folly is to people the world with fools.”