What is an API?

An Application Programming Interface or API is what allows one software application to "talk" to another. For example, imagine connecting your calendar and to-do application and keeping them in sync; you need to work with their APIs. Most modern software applications and almost the entire Web is made up of APIs.

In a nutshell, an API is just a set of rules (protocol).

The protocol allows two software systems to be linked regardless of differences in their internal processes, structure, or design. The most common protocol for building APIs is HTTP.