If you've pulled data from an API, you know how useful they are for retrieving information. But if you need that data pushed to your app as it happens, webhooks are even better. Webhooks are an event-driven message sent to a URL of your choice, and they are offered by many services that have an API.
Development Blog: API
User registration using Drupal 8's REST API has been in flux but appears to be stabilized (for now).
There are many blog posts on using Drupal 8's REST API. Getting a user registered using REST, however, seems to be in flux. This post will get you set up for using the REST API, and in particular, will outline the steps to get a new user registered on a Drupal 8.3.x site.
First, enable the 4 core Web Services modules: HAL, HTTP Basic Authentication, RESTful Web Services (REST), and Serialization.