The WordPress REST API is a relatively new addition to the WordPress platform. Before it was added to core, the REST API was available as a standalone mechanism via a separate plugin. You could get everything and also learn how to work with the package from the project's official website at http://v2.wp-api.org/.
However, the REST API was included in the WordPress core package in WordPress version 4.7. This means that you no longer need any additional plugins or tools to make it work. Basically, as long as you're on the most recent version of WordPress, you have the REST API installed and enabled by default. Which is great news! But let's get a bit deeper.
