In the wide and varied family of the thousands and thousands of funds out there, the exchange-traded fund is one of the more consumer-friendly ones.

Unlike mutual funds, exchange-traded funds, or ETFs, behave more like stocks. You can buy into an ETF at any time, and sell it whenever you feel like it. And like a stock, an ETF's value can rise and fall--depending on what the ETF is invested in.What do ETFs invest in? Well, they're typically linked to an index like the Dow Jones Industrial Average or the S&P 500. So, if you had an ETF that trades the same companies that make up the Dow or the S&P, it will rise and fall in value pretty much the same amount as the Dow or S&P.

You can also buy ETFs that invest in other types of products, like bonds, currencies, gold or other commodities. The ETF market has grown considerably in the past few years, so there is no shortage of ETFs to invest in.