Stormpath made a name for itself with its backend, REST API that provides registration and authentication. The REST API operates in the backend of an app. As serverless architecture grows in use, a need for a frontend registration and authentication services arose. Enter the Stormpath Client API.
There are three standard ways to manage API authentication these days: API keys, OAuth tokens and JSON Web tokens (JWT). Adam Duvander over at the Zapier engineering blog explains how and when to use them. The humble API key is the common and earliest form of API authentication.