Exchange-traded funds (ETFs) are an investing innovation that combines the best features of index mutual funds with the trading flexibility of individual securities. ETFs offer diversification, low expense ratios and tax efficiency in a flexible investment that can be adapted to suit a multitude of objectives. (To learn more about the features and benefits that ETFs offer, see Advantages of Exchange-Traded Funds.) To reap the true benefit of investing in ETFs you need to use them strategically. 

At the most basic level, ETFs can be used as part of both long-term and short-term investment strategies. Their low expense ratios and high trading flexibility make them attractive alternatives to traditional mutual funds.

Index Investing
From a strategic standpoint, the first and most obvious use of ETFs is as a tool to invest in broad-market indexes. On the equity side, there are ETFs that mirror the S&P 500, the Nasdaq 100, the Dow Industrials and just about every other major market index. On the fixed-income front, there are ETFs that track a variety of long-term and short-term bond indexes including the Lehman 1-to-3 Year Treasury, the Lehman 20-Year Treasury and the Lehman Aggregate Bond Index. (Again, if you are not familiar with the broad range of market index coverage that ETFs offer, see Advantages of Exchange-Traded Funds for more information.)

Using ETFs to cover the major market sectors, you can quickly and easily assemble a low-cost, broadly diversified index portfolio. With just two or three ETFs, you can create a portfolio that covers nearly the entire equity market and a large portion of the fixed-income market. Once the trades are complete, you can simply stick to a buy-and-hold strategy as you would with any other index product, and your portfolio will move in tandem with its benchmark.

Actively Managing a Longer-Term Portfolio
In a similar fashion, you can create a broadly diversified portfolio but choose a more active-management strategy instead of simply buying and holding to track the major indexes (which is passive management). While the ETFs themselves are index funds (meaning there is no active management on the part of the money manager overseeing the portfolio), this doesn't stop investors from actively managing their holdings. For example, say you believe that short-term bonds are set for a meteoric rise; you could sell your position(s) in the broader bond market and instead buy an ETF that specializes in short-term issues. (You could do the same for your expectations for equities.)