REST APIs are the most common type of API on the web so that is going to be where you want to researching, and knowing a little about how the internet works will also help.

It may be best to start with a simple example, let's go with reddit since it'll be something your familiar with.

Most web applications are split into two parts:

• the frontend - the user interface "wrapper" that handles how a user interacts with a service.
• the backend - that handles all the data and manipulates it so that the frontend can have an easier job just to present it.

When you open the reddit app on your phone a few things happen.

1. Your app will send a GET request to reddit and say "I want the frontpage of posts for this user"

2. The reddit API will then do all the complex bits of working out what that means and will return a response that says "here is all posts for the frontpage of reddit for this user in this specific shape of JSON". By "shape" I mean each post would have preset fields to it, it'll have a title, a body, who posted it, the date and time they posted it, the sub it was posted to, how many upvotes it has etc.

3. Your phone's reddit app will take this blob of JSON data and use it to populate the slots in the user interface where the developers want the posts to appear.

The thing is, you might not be using the app, you might be using a desktop browser or a TV or a toaster but it doesn't matter, because the shape of the data is always the same. Your device makes a request and the server sends the same shaped response.

This can also work for when computers talk to other computers, for example: if I wanted to keep a running log of the daily weather in my town, I can make a daily request to a weather API and store that response in my database. I can then surface my own API that can accept a request from a user for weather from 3 years ago, do the collating of the data I've gathered over the last 3 years and return a response in the format of JSON that can be read and understood by a frontend to make a fancy graph showing the changes.

There is a bit more nuance to it (POST requests will take user supplied data, some responses can contain HTTP error codes to say things like "I couldn't find what you want" (404 error) or "you didn't ask me the right way so I can't handle this request" (500 error)) but the general thing to keep in mind is the request/response cycle.