APIs (sometimes described as web services) work in the background whenever two applications interact, from sending instant messages to making purchases through online ticketing systems etc. This all happens
behind the scenes, making user experiences effortless.

In an internet-connected world, web and mobile applications are designed for humans to use, while APIs are designed for other digital systems and applications to use. Websites and APIs both do the same things, like return data, content, images, video, and other information. But APIs don’t return all the details that are needed to make things look pretty for the human eye.
We only get the raw data and other machine-readable information needed behind the scenes to put the resources being delivered to work, with very little assistance from a human.

API integration is simply the connection between two (or more) applications, programs, services, or systems, using APIs. Applications use APIs to send and receive data and content between each other.