A stock market crash is a sudden dramatic decline of stock prices across a significant cross-section of a stock market. Crashes are driven by panic as much as by underlying economic factors. They often follow speculative stock market bubbles. Stock market crashes are social phenomena where external economic events combine with crowd behavior and psychology in a positive feedback loop where selling by some market participants drives more market participants to sell. http://en.wikipedia.org/wiki/Stock_market_crash
The Dow is down over 30% YTD as of this writing. Pretty bad if you used all the gains from selling your house and put it into the stock market this time last year.