Buying a security such as stocks on margin is a form of leverage. An investor uses funds borrowed from a brokerage company to purchase the equities. The Federal Reserve Board governs margin requirements for brokers such as Merrill Lynch and Wachovia.
Assume an investor contributes 50% to the purchase of $10,000 worth of a stock. The investor sends $5,000 to his brokerage firm and the brokerage firms lends him the additional $5,000 needed for $10,000 total. The term "investor" no longer makes sense because now that person has crossed the line and has become a "speculator" hoping to magnify his gains. The next day the share price falls and his speculative portfolio is worth only $4,000. But the speculator owes the brokerage firm $5,000. Ruh Roh! The speculator gets the dreaded margin call.
A margin call is a demand from a broker for additional cash or securities to bring a margin account back within minimum maintenance limits. Brokerage firms are forced to liquidate holdings if the minimum margin requirements are not met. In the above example, the brokerage firm will likely liquidate the person's holdings and demand an additional $1,000 worth of cash or securities to remain whole.
Hence the vicious spiral down in the equities market. I heard they were going to remove the word 'buy' from the dictionary due to lack of use. I would not reinvest in the markets until all leveraging has been purged.