If you ever traded stocks and paid attention to the financial news, you must have heard of the VIX. It is a "doohickey" that is supposed to measure volatility of the market, i.e. risk.
Here is the official Cboe definition (http://www.cboe.com/products/vix-index-volatility/vix-options-and-futures/vix-index/vix-faqs#2). But this paper gives you a wonderful insight on how to understand that index. And here is a wonderful explanation of how to do the math as well.