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 investors 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, among other things, to file quarterly earnings reports with the Securities and Exchange Commission (SEC). 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, (Fidelity Investments parent) FMR Corp., 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, Purdue Farms, McKinsey & Co., and LEGO Company. Some of these firms offer shares to employees, but individual investors are out of luck.

Domino's Pizza (NYSE:DPZ) was private until recently, as were Goldman Sachs (NYSE:GS) and United Parcel Service (NYSE:UPS). 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.

If you'd like to make money in stocks, you'll need to stick to public companies. If you'd like to receive several promising stock ideas delivered via email each month, learn more about our suite of investment newsletters (which are offered along with some free research reports). Their performance may surprise you. You can also learn all about brokerages and find one that's right for you in our Broker Center. (Did you know that some well-regarded brokerages are offering commissions as low as $5?)