Using Auth0 in your applications means that you will be "outsourcing" the authentication process to a centralized login page in the same way that Gmail, YouTube, and any other Google property redirects to accounts.google.com whenever a user signs in.
Your user will authenticate, and Auth0 will generate an ID Token and Access Token that will be passed back to your application. The Access Token can then be used to call your API.
When your app needs to fetch user data from your API:
For single-page web apps, Auth0 uses the Implicit Flow.
The easiest way to implement the Implicit Flow is to follow our Single-Page App Quickstarts.
You can also use our Auth0.js SDK.
Finally, to use our API endpoints, you can follow our tutorial: Call Your API Using the Implicit Flow.
Step-by-step instructions for tasks
Understand the fundamentals