Auth0 is a service that abstracts how users authenticate to applications.
You can connect any application (written in any language or on any stack) to Auth0 and define its Connection, the method used to authenticate the users of that application:
Custom credentials: username + passwords
Social network logins: Google, Facebook, Twitter, and any OAuth2, OAuth1 or OpenID Connect provider
Enterprise directories: LDAP, Google Apps, Office 365, ADFS, AD, SAML-P, WS-Federation, etc.
Passwordless systems: TouchID, one time codes on SMS, or email