The current global credit crunch started with subprime mortgages.
Banks lent money to people, often with a higher value of the properties which they intended to use the loans for. Loans were offered at attrative fixed rates and not long after that, reverted to the market-floating rates after a few years. The interest rates rose and the prices of properties slumped, making the customers who borrowed the loans, not being able to pay back the creditors resulting in defaults. The number of defaultors rose and banks could not receive the back the money which they rent. Reserves of cash fell low and the banks were squeezed.