The App.net Ruby SDK by App.net communicates with the API to implement authentication and identity capabilities into cross-platform applications. Installation includes ruby gem. Usage provides token to make API calls.
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.