When a corporation is formed, it issues stock (股票), which is sold or given to individuals. Ownership of stock entitles you to vote in the election of a corporation's directors, so in theory holders of stock control the company. In practice, however, in most large corporations, ownership is separated from control of the firm. Most stockholders have little input into the decisions a corporation makes. Instead, corporations are often controlled by their managers, who often run them for their own benefit as well as for the owners. The reason is that the owners' control of management is limited.
A large percent of most corporations' stock is not even controlled by the owners; instead, is controlled by financial institutions such as mutual funds (financial institutions that invest individuals' money for them) and by pension funds (financial institutions that hold people's money for them until it is to be paid out to them upon their retirement). Thus, ownership of I Corporations is another step removed from individuals. Studies have shown that 80 percent of the largest 200 corporations in the US are essentially controlled by managers and have little effective stockholder control.
Why is the question of who controls a firm important? Because economic theory assumes the goal of business owners is to maximize profits, which would be true of corporations if stockholders made the decisions. Managers don't have the same motivation to maximize profits that owners do. There's pressure on managers to maximize profits, but that pressure can often be weak or ineffective. An example of how firms deal with this problem involves stock options. Many companies give their managers stock options-rights to buy stock at a low price - to encourage them to worry about the price of their company's stock. But these stock options dilute (稀释) the value of company ownership and decrease profits per share and can give managers an incentive (激励,刺激) to overstate profits through accounting tricks, as happened at Enron, Xerox, and a number of other firms.