The iShares Russell 2000 Growth Fund (IWO) is an exchange-traded fund that consists of some of the fastest-growing companies on Wall Street. More specifically, the fund tracks a basket of roughly 1,300 of the 2,000 stocks that comprise the Russell 2000 SmallCap Index. These 1,300 companies are expected to grow their revenue and earnings at an above-average clip going forward. Of course, as you would expect, they also sport among the highest price-to-book ratios in the index. (You can calculate a stock's price-to-book ratio by dividing the firm's share price by the company's book value per share.)

These fast-growing smallcap companies are typically among the most volatile on the market. They often do not have strong -- or in some cases even any -- earnings. Their beta when compared to the stock market is usually well above 1.00. (Note: Beta measures how much a stock moves when compared with a broad stock market index such as the S&P 500. For example, a stock with a beta of 2.00 will tend to rise +20% when the S&P increases +10%.)

Unlike many other ETFs out there, no single stock accounts for a large chunk of IWO's value. For example, the top holding in the fund, RF Micro Devices (RFMD), does not even account for 0.5% of the fund's value. However, IWO is weighted heavily (24%) in technology issues and has a strong correlation with the Nasdaq Composite and the Nasdaq-100 indices.