A public company is listed on a stock exchange and anyone who has enough money can buy a share of that company.
In order to be listed on a stock exchange in America companies must submit business and financial information to the exchange and have their financial information audited by a CPA. For years Anheuser Busch stock was sold on the American stock exchange. The New York Stock exchange required that Busch reveal their secret formula for brewing beer. Busch refused and therefore their stock could not be sold on the New York stock exchange.