Leveraged exchange-traded funds, or ETFs, can effectively double or triple your exposure to a certain index or asset class and can be used to create a long (bull) or short (bear) position. For example, a triple-leveraged S&P 500 ETF will return three times the daily performance of that index. However, before you buy a triple-leveraged ETF, it's important to know how they work -- and the drawbacks of holding them for long periods of time.To illustrate what these investment vehicles are and what they might be used for, here are three examples of popular triple-leveraged ETFs.