http://stackoverflow.com/questions/2171177/what-is-application-binary-interface-abi
ABIs cover details such as
- data type, size, and alignment;
- the calling convention, which controls how functions' arguments are passed and return values retrieved;
- the system call numbers and how an application should make system calls to the operating system;
ABIs may be defined at the processor-architecture level or at the OS level. the ABIs are standards to be followed by the code-generator phase of the compiler. the standard is fixed either by the OS or by the processor.
functionality: define the mechanism/standard to make function calls independent of the implementation language or a specific compiler/linker/toolchain. provide the mechanism which allows JNI, or python-c interface etc