What is it?
Core Foundation (also called CF) is a C application programming interface (API) and is a mix of low-level routines and wrapper functions. Apple releases most of it as an open source project called CFLite that can be used to write cross-platform applications for Mac OS X, Linux, and Windows; a third-party open-source implementation called OpenCFLite also exists.
Description of corefp.dll from Apple
“Core Foundation is a framework that provides fundamental software services useful to application services, application environments, and to applications themselves. Core Foundation also provides abstractions for common data types, facilitates internationalization with Unicode string storage, and offers a suite of utilities such as plug-in support, XML property lists, URL resource access, and preferences.”