Antitrust laws prevent companies from taking over an industry sector and thus stifling fair trade within that sector. A single company that dominates a sector without competition is called a monopoly. Generally, monopolies are not good for economies, as they reduce choices and increase prices for consumers.
Antitrust laws also prevent mergers that will result in less choice and competition. One of the most famous antitrust cases is that of Microsoft vs. The United States. Charges of antitrust were brought against Microsoft because of its Internet Explorer web browser, which was installed on Windows PCs by default. The court ruled that Microsoft constituted unlawful monopolization. Microsoft appealed the case and eventually reached a settlement.