API (Application programming Interface) is a tool for developers. It helps them to building their application by using resources built by others. For instance a developer can use  Google Web Search API  to use search results within her application or use Google Maps API . Image Source The essential purpose of API to help developer harness resources available elsewhere. An API is means by which third parties can develop code that can interface with other code written by other developer. API may consists of - a set or routines, data structures, object classes, protocols that is provided by libraries or operating system services- all meant to be used in building of applications.  Examples: Doubleclick have a published Java API that allows users to interrogate the database tables to get information about their online advertising campaign  The Amazon Product Advertising API provides programmatic access to Amazon’s product selection and discovery functionality so that