To follow up on Joe's Comment, and just to give a feel for the size of the money involved. One widely quoted index is the S&P 500. This is based on the market capitalisation (what it would cost to buy the shares) of 500 large US companies from various sectors. These companies all together have a market capitalisation of over $9 trillion! The S&P index was created in 1957, and currently stands at a value around 1500. So on that index, every point loss represents a percentage fall of about 0.067%. That doesn't sound much, but it represents a movement of $6 billion in the market values of those companies. And of course the index is just a sample of all the shares traded in all the companies around, so the 'real numbers' are bigger.

cheers KP