Basically a meltdown of the credit system. Too many home loans were written without enough equity. Eventually, housing prices started to fall, spurred by adjustable rate mortgages whose payments increased substantially and which the borrowers could no longer afford to pay. This threw a lot of foreclosed housing on the market, further depressing prices. Numerous financial institutions all over the world were holding mortgage paper, and a lot of it went bad. This eroded the banks' capital positions, so that they could no longer make loans ti anybody. Modern business is crucially dependent on credit to function, and it was necessary to inject much new liquidity into the banking system to keep business from grinding to a halt.