What is an index fund? Well, to start, it’s the type of investment recommended by the most famous investor in the world. On December 19, 2007, Warren Buffett, chairman of Berkshire Hathaway and one of the world’s wealthiest investors, made a bold bet with hedge fund manager Ted Seides. He wagered that a low-cost S&P 500 index fund would outperform a portfolio of actively managed hedge funds…