Application Program Interfaces, (more commonly known as "APIs"), are used to connect two different databases or software programs.
APIs are a set of routines, protocols, and tools that are used for building software applications. APIs specify how software components should interact and are used when programming graphical user interface components. A good API makes it easier to develop a program by providing all the building blocks for a programmer, who then has to put the pieces together. There are multiple different types of API’s for operating systems, applications or websites. Most operating environments provide API’s that allow programmers to write applications consistent with the operating environment.
An example of a popular "open"API is Twitter. Twitter uses two types of API’s, the first one is the REST API, which allows developers to access core Twitter data. The second is the Search API, which provides a method for developers to interact with Twitter Search and trends data.