API stand for (Application Programming Interface) which is collection of prewritten packages, classes, and interfaces with their respective methods, fields and constructors. It is work as an user interface between humans and computers. An API working as a software program which is provide interface service interaction. In C# , ASP.Net and Java most basic programming tasks are performed by the API’s classes and packages and these are helpful in minimizing the number of lines written within pieces of code. In these APIs many components are pre created and commonly used. All the programmer use these prewrittin code via APIs. After referring to the available API classes and packages, the programmer easily invokes the necessary code classes and packages for implementation.