What is API?
API stands for Application Programming Interface. It is a software specially designed to allow communication between two or more applications. For example, when you use a mobile phone to pay for something, an application on your phone uses an API to communicate with a bank server.
What are the types of Web API?
There are many types of Web APIs:
Open,
Internal,
Partner and
Composite API.
API types
Open APIs,
Partner APIs,
Internal APIs and
Composite APIs.
Which are API protocols?
REST,
JSON-RPS and XML-RPC,
Graph Querry Language GraphQL and
SOAP.
Which protocol is used the most?
REST API is the most popular protocol used in the software industry.
How API works?
REQUEST method (GET, PUT, HEAD, POST, PATCH, CONNECT, TRACE, OPTIONS and DELETE). A client application initiates an API call to retrieve information—also known as a request. This request is processed from an application to the web server via the API’s Uniform Resource Identifier (URI) and includes a request verb, headers, and sometimes, a request body.
CALL. After receiving a valid request, the API makes a call to the external program or web server.
RESPONSE. The server sends a response to the API with the requested information.
SEND. The API transfers the data to the initial requesting application.
References
https://www.spiceworks.com/tech/devops/articles/application-programming-interface/#_004
https://www.numpyninja.com/post/basics-of-api-and-api-protocols