An Application Programming Interface (or commonly abbreviated as API), is a mechanism for exposing the core functionality of an application (such as a client or desktop program, web site or web service) to an external application (of any of the previously mentioned types).
Since the days of Web 2.0, an API is seen as a crucial element to any Web Application or Web Service. In general though, APIs are crucial parts of an application design and implementation strategy. They ensure the involvement of third-parties and outside developers in the products and services you create, and they can also help to breed innovation.
<script> tag to be added to a webpage in order to expose their functionality. For example: