Carbon is one of Apple Inc.'s C-based application programming interfaces (APIs) for the Macintosh operating system. Carbon provides a good degree of backward compatibility for programs that ran on the now-obsolete Mac OS 8 and 9. Developers could use the Carbon APIs to port their "classic" Mac software to the Mac OS X platform with far less effort than a port to the entirely different Cocoa system which originated in OpenStep.

Carbon was an important part of Apple's strategy for bringing Mac OS X to market, offering a path for quick porting of existing software applications. As the market has increasingly moved to the Cocoa-based frameworks, especially after the release of iOS, the need for a porting library was diluted. Apple did not create a 64-bit version of Carbon while updating their other frameworks in the 2007 time-frame, and eventually deprecated the entire system in OS X 10.8, which was released on July 24, 2012.

Site: http://developer.apple.com/carbon/