API stands for application program interface. A programmer writing an application program can make a request to the Operating System using API (using graphical user interface or command interface). It is a set of routines, protocols and tools for building software and applications. It may be any type of system like a web-based system, operating-system or a database System.
API in Procedural languages
It specifies a set of Functions and Routines that helps in completing a task.
API in object-oriented languages
It simply shoes how an Objet work in a given Object-Oriented language. It is expressed as a set of classes with an associated list of class Methods.
API in libraries and Frameworks
It is related to Software libraries and Software Framework.
API and Protocols
It can be implementation of Protocols.
API sharing and reuse via virtual machine
Languages running on virtual machines can share can share an API.
Several principles are used in designing of API’s. One method is the concept of information hiding. It works on the principle that a software can be divided into modules, each of which has a specified interface.
Another is software architecture which is used to create and maintain high-level software structures.