API stands for Application Programming Interface, is a set of routine definitions, protocols, library, and tools that assists developers in writing code that connect with other software. It describes what functionality is available, how it must be used and what formats it will accept as input or return as output. An API may be for a web-based system, operating system, or database system, and it provides facilities to develop applications for that system using a given programming language.
The API allows programmers to use predefined functions to interact with the operating system, instead of writing the code from scratch. Now a days developers no longer have to build a core application that tries to do everything. Instead, they can contract out certain specific responsibilities by using already created pieces of codes(APIs) that do their job better. All computer operating systems, such as Windows, Unix, and the Mac OS and language such as .Net, Java etc. provide an application program interface for programmers. Almost every application depends on the APIs of the underlying operating system to perform such basic functions as accessing the file system. In essence, a program's API defines the proper way for a developer to request services from that program.
As more and more companies have adopted the Web as the primary network for systems integration and have started seeking ways to connect their IT assets to websites and mobile apps, adoption of APIs has grown rapidly. APIs allows organizations to exposing defined assets, data, or services for public consumption. That makes it possible for applications to share data and take actions on one another's behalf without requiring developers to share all of their software's code. For example, Amazon.com released its API so that Web site programmers could more easily access Amazon's product information. Using the Amazon API, a third party Web site can post direct links to Amazon products with updated prices and an option to "buy now".
Popular API Examples
Broadly speaking, APIs make it possible for organizations to open their backend data and functionality for reuse in new application services. Google has APIs for search, calendars, translations, etc. Facebook and Twitter have APIs that allow software to automatically post status updates. Moreover, APIs have become so valuable that they comprise a large part of many business' revenue. Major web development companies like Google, eBay and Amazon are just a few of the companies that make money from their APIs.
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