The Carbon APIs can be used to write Mac
OS X applications that also run on previous versions of the
Mac OS (8.1 or later).
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.
Developers interested in Carbon should begin by reading the Inside
Mac OS X: System Overview (pdf) book to understand the overall
system architecture
and how Carbon fits into the picture.