Hedge Funds Statistics

The NY Times reports that one of the largest Hedge Funds is under investigation by the S.E.C. I noticed a few statistics on the size and influence of Hedge Funds:

... Hedge funds control an estimated $1.2 trillion in assets currently, almost 3,000 percent more than they did 16 years ago. Last year, almost 2,100 hedge funds opened for business. Despite their growth, hedge fund assets total much less than the $9.2 trillion invested in mutual funds domestically.

But while hedge fund represent only 5 percent of assets under management in the United States, they account for 30 percent of stock trading volume nationwide. As a result, hedge funds can have an enormous impact on the nation's securities markets.

How do they define a "Hedge Fund": there are so many types of investment vehicles and trading styles, that some clarification would be helpful. Secondly, stock trading volume is one measure, but the amount of leverage will probably have more of an impact if and when a financial crisis occurs.