Definition

API is a set of definitions and protocols for building and integrating application software. It is referred to as a contract between an information provider and information user:

1. Call - establishes content required from consumer.
2. Response - content required by the producer.

Think of an API as a mediator between the users or clients and the resources or web services they want to get. It's also a way for an organization to share resources and information while maintaining security, control, and authentication - determining who gets access to what.