API (or Application Programming Interface) is a set of routines and standards set by software for the use of its features for applications that do not intend to engage in the implementation details of software, but only use their services.
In general, the API comprises a number of functions accessible only by programming and enabling use less evident characteristics of the user software traditional.
For example, an operating system has a lot of functions in the API, which allow the programmer to create windows, access files, encrypt data etc.. But the APIs of the operating systems are usually tied to the most essential tasks, such as the manipulation of blocks of memory and device access. These tasks are the core attributes and system rarely are programmable. Another example is geometric design programs that have a specific API to automatically create entities in accordance with standards set by the user.