An API is a tool that allows data to be retrieved from one system by another system or application.  The data can then be used in other websites, applications, compiled into reports, for your own data analysis, and more.  APIs are all about connection and information sharing - integrating data from one source to another. 

APIs were created to satisfy a demand for access to data. The more APIs that are developed, the more the appetite for API consumption has grown.  APIs enable website, web, and mobile software to build applications that were not possible before APIs.