Hi I, They mirror some index like the Dow Jones U.S. Energy Index or some other. So they are passively managed by owning in essentially the same ratios the equities of that index. Very low turn-over and very low annual costs.
At this site http://quotes.nasdaq.com/asp/ETFsSector.asp you can click on the sector of interest and read about the index fund. Many show the top ten holdings and the industries by percentage.
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