Some 38 million Americans have invested more than $2 trillion in mutual funds. These investment pools have been around for years, offering small investors an opportunity to enter the stock and bond markets alongside wealthy individuals, pension funds and insurance companies. But never have mutual funds garnered the kind of attention -- and money -- that they have in the past few years. Lured by the promise of high returns at a time when interest rates on bank certificates of deposit were low, many consumers switched to mutual funds as an alternative means of saving for such big items as the down payment on a house, college tuition and retirement. Some economists worry, however, that newcomers to mutual funds may not fully understand the risks involved.