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  • Giovanni Maria Casucci thinks this is interesting:

The second difference is that Azure AD and on-premises AD DS use different protocols. On-premises AD DS uses protocols such as Kerberos and Lightweight Directory Access Protocol (LDAP), whereas Azure AD uses Internet-oriented protocols, such as SAML 2.0, ws-Federation, OpenID Connect, and RESTful Graph API.

From

Cover of Exam Ref 70-534 Architecting Microsoft Azure Solutions

Note

On Premises AD uses LDAP or Kerberos
Azure AD uses SAML 2.0, ws-Federation, OpenID Connect