Most firms in our economy are small sole proprietorships with only one owner, or partnerships with several owners. Corporations , which are usually owned by many stockholders, are much less common than sole proprietorships and partnerships, but corporations produce most of the output and receive most of the revenues in the economy.
Economists usually call firms small or large relative to their markets.
So far, we've been assuming that firms charge the same price to all their customers. In some real-world markets, firms use price discrimination to charge different customers different prices. Also, in some real-world markets, firms are interdependent. That is, the actions of one firm have a noticeable effect on other firms, and these other firms are likely to react. Interdependence makes it more difficult for firms to choose a profit-maximizing pricing strategy.