What is an API?

API is the acronym for Application Programming Interface, which is a software intermediary that allows two applications to talk to each other. 

Each time you use an app like Facebook, send an instant message, or check the weather on your phone, you’re using an API.

The Modern API

Over the years, what an “API” is has often described any sort of generic connectivity interface to an application. More recently, however, the modern API has taken on some characteristics that make them extraordinarily valuable and useful:

• Modern APIs adhere to standards (typically HTTP and REST), that are developer-friendly, easily accessible and understood broadly
• They are treated more like products than code. They are designed for consumption for specific audiences (e.g., mobile developers), they are documented, and they are versioned in a way that users can have certain expectations of its maintenance and life cycle.
• Because they are much more standardized, they have a much stronger discipline for security and governance, as well as monitored and managed for performance and scale
• As any other piece of productized software, the modern API has its own software development life cycle (SDLC) of designing, testing, building, managing, and versioning.  Also, modern APIs are well documented for consumption and versioning.

In other word an API is a standard method to communicate with an application

Most APIs have a very detailed documentation about how to use they, so it is always useful the search for the API documentation, whenever you want to use it.