Key Takeaways:

The WordPress REST API is a technology that exposes each WordPress site's data to be accessed and changed by external applications: applications that can be written in any language, and can be located on any device or server.
All REST APIs, including the WordPress REST API, work with data using verbs (commands) that are intentionally similar to HTTP verbs. The main verbs used by the WordPress REST API are: GET, for accessing data, POST, for changing data, and DELETE, for deleting data.
The WordPress REST API works via four key concepts: routes, the URLs where we access data, resources, the data we've got to work with, verbs, which dictate what to do with the resources, and endpoints, which tie the four concepts above together into a single action.

As of WordPress 4.7 (which shipped in late 2016), the WordPress REST API is a built-in part of WordPress core. It was one of the mo...