The CFFI library is composed of a Lisp-implementation-specific backend in the CFFI-SYS package, and a portable frontend in the CFFI package.
The CFFI-SYS backend package defines a low-level interface to the native FFI support in the Lisp implementation. It offers operators for allocating and dereferencing foreign memory, calling foreign functions, and loading shared libraries. The CFFI frontend provides a declarative interface for defining foreign functions, structures, typedefs, enumerated types, etc... It is implemented in portable ANSI CL making use of the low-level operators exported by CFFI-SYS.
A UFFI compatibility layer is also included.
More information about CFFI (including documentation) is available in CFFI's home page at common-lisp.net.
A companion project is Verrazano, a FFI generator (from C and C++ headers) that targets CFFI.
UNIX systems have CFFI wrappers at cffi-posix