1) What is API?
An API (Application programming interface) is a software intermediary that allows two or more applications to talk to each other.
2) API type in terms of release policies
Private: It can be used within the organisation.
Partner: It can be used within business partners.
Public: It can be used by any third-party developers.
3) How to use API? (just an overview)
Using an API means asking another system for things you need, showing a secret pass (token) to get what you want, and then using that stuff in your work.