'Carbon' is one of two development environments in Mac OS X(10). This process allows a developer to port their Mac OS 9 apps to OS X with a minimal of code change.
"Carbon is designed to provide a gentle migration path for developers transitioning from Mac OS 9 to Mac OS X. As most Carbon APIs are supported on Mac OS 9, developers can enhance applications for Mac OS X while continuing to support customers who are using previous versions of Mac OS. Carbon also allows Mac developers to use the software development tools and languages they are most familiar with to develop for Mac OS X."
The second development environment is Cocoa. Inherited from NeXT it uses and object oriented programming interface via Objective C, several developers have exclaimed how robust and fast this environment is. Allowing them to create large apps several times faster than with C++.
For those of you who don't know OS X is based on UNIX. The kernel is composed on Mach and FreeBSD. Offering stability, resource management, and flexibility on a server level. At the same time wrapped in a easy to use and pleasing to the eye GUI called Aqua.