APIs, or Application Program Interfaces, provide the means for different systems to communicate with each other. The API can limit the number of connections or queries and access to the data, which is good for the person hosting the data, and provide a standard means of communication for the developers trying to make something interesting out of the data. This only makes sense as big data continues to grow and more people and objects gain sensors to be tracked and analyzed.
This infographic goes into some APIs that you could encounter on a daily basis and how some of their functionality is innovating the world in new ways.
(Credit: whoishostingthis.com)