An Application Programming Interface (API) is a public persona for a company, exposing defined assets, data, or services for public consumption. An API is a way for services and products to communicate with each other through a documented interface. APIs allows companies to open up data to external third party developers, to business partners and internal departments within their company. An app developer can leverage an API with ease and invoke it via a web browser, mobile application, or device. Product catalogs, phone listings, insurance cases, order status, and bank loan rates are a few of the services exposed via APIs.
APIs are different from websites. The content and the layout of the website is dynamic and change in the content does not affect the website visitors whereas any changes to APIs need to be informed to the developers using the API as it affects the applications built on the APIs. This does not mean that there will be no changes in an API. An API constantly changes to include new functionalities based on business requirements and developers are informed of these changes. Once established, APIs are more of a product than a mere technical capability.
To see a real use case, visit the Acme Freight API Journey.