A Classic application is one for which the original system requirements, normally printed on the application's packaging, include Mac OS 9 but not Mac OS X. The system requirements listed may be "System 7.6.1 to Mac OS 9," for example. These applications cannot be used natively in Mac OS X, which is to say they do not work in the absence of a Mac OS 9 installation and the Classic environment.
Mac OS X includes the Classic environment. It uses a copy of Mac OS 9 to make Classic applications work in Mac OS X. Both Mac OS 9 and Mac OS X must be installed on the computer. The Classic environment starts automatically when you open a Classic application.
Applications that list both Mac OS 9 and Mac OS X in their system requirements are often Carbon applications. Carbon is a type of application that may work natively in either Mac OS 9 or Mac OS X. Neither the Classic environment nor a Mac OS 9 installation is needed to use a Carbon application in Mac OS X.
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