Ever wonder what the difference is between a private and public company? A private company is one that's privately owned -- usually by one or a few people. Its owners don't have to reveal much about their business. And most of us can't invest in it.

A public company is one that has sold a portion of itself to the public, via an initial public offering (IPO) of some shares of its stock. Therefore, it probably has hundreds or thousands of co-owners. If it's an American company trading on American stock exchanges, it's required to file quarterly earnings reports with the Securities and Exchange Commission (SEC), amount other things. These are also made available to shareholders and the public. A public company can't keep mum about how much it made in sales last year. It must report information like that -- its revenues, cost of sales, tax expenses, administration costs, debt load, cash level, and so on.

Here are a bunch of major companies that remain private: Cargill, Subway, M&M Mars, Bertelsmann AG, Bechtel, Publix Super Markets, IKEA International, FMR Corp. (parent of Fidelity Investments), Seiko, Epson, Amway, DHL Worldwide Express, Virgin Group, Rosenbluth International, Penske, S.C. Johnson & Sons, Enterprise Rent-A-Car, Hallmark Cards, Borden, McCain Foods, Hyatt, National Amusements, PurdueFarms, McKinsey & Co., LEGO Company, and Domino's Pizza. Some of these firms offer shares to employees, but individual investors are out of luck.

United Parcel Service (NYSE:UPS) was private until recently, as was Goldman Sachs (NYSE:GS). Levi Strauss, once private, went public only to go private again. Just because an old, established firm is private doesn't mean it'll always be private.

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