At the most innocuous level, the term is applied to the legal and mundane methods of examining corporate publications, web sites, patent filings, and the like to determine the activities of a corporation (though this is normally referred to as business intelligence), through to bribery, blackmail, technological surveillance and even occasional violence. As well as spying on commercial organisations, governments can also be targets of commercial espionage - for example, to determine the terms of a tender for a government contract so that another tenderer can underbid.
Most large corporations openly acknowledge the existence of departments to perform the legal aspects of corporate espionage. Many also spend considerable amounts on precautions to protect against the more cloak-and-dagger varieties.
The United States government has admitted to using commercial espionage, for instance using surveillance of phone calls to determine that a French competitor of a US firm was bribing Brazilian officials to obtain an air traffic control radar contract (it was later revealed that the US firm was *also* bribing officials). It is generally believed that most large intelligence agencies are involved in the practice.
The development of the Tupolev Tu-144 supersonic aircraft, with it's rapid design and similarity to Concorde, was one of the most prominent examples of industrial espionage in the 20th century.