Apple's programming interface (API) for the Mac OS X operating system. Formerly known as the Yellow Box and derived from the OpenStep environment from NeXT, Cocoa applications are created with Apple's Xcode and Interface Builder development environments as well as with languages such as Perl and Python. See Xcode and OpenStep.

Carbon and Classic for Compatibility
Carbon is the programming interface (API) for writing applications that run on both Mac OS X and earlier Mac operating systems, while Classic was the API for the Mac prior to OS X. Classic also refers to the Blue Box, which is the OS X system program that runs a legacy Mac application as is. See Mac OS X.