An ABI defines how data structures or computational routines are accessed in machine code, which is a low-level, hardware-dependent format; in contrast, an API defines this access in source code, which is a relatively high-level, relatively hardware-independent, often human-readable format. A common aspect of an ABI is the calling convention, which determines how data is provided as input to or read as output from computational routines; examples are the x86 calling conventions.
Adhering to an ABI (which may or may not be officially standardized) is usually the job of a compiler, operating system, or library author; however, an application programmer may have to deal with an ABI directly when writing a program in a mix of programming languages, which can be achieved by using foreign function calls.
the calling convention, which controls how functions' arguments are passed and return values are retrieved; for example, whether all parameters are passed on the stack or some are passed in registers, which registers are used for which function parameters, and whether the first function parameter passed on the stack is pushed first or last onto the stack
how an application should make system calls to the operating system and, if the ABI specifies direct system calls rather than procedure calls to system call stubs, the system call numbers
and in the case of a complete operating system ABI, the binary format of object files, program libraries and so on.