I develop a SharePoint Online Provider hosted add-in.
My question is, does SharePoint Online use the Azure AD to authenticate the Users, Or is there any extra configuration neccessary for this?
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Yes, SharePoint Online uses Azure AD (or the Azure ACS to be precise) for authenticating users (if no ADFS is put in place for trusting your on-premises AD to authenticate users that are synchronized from the same, but you will still be able to use authentication provided for Azure ACS, like ADAL.js, to authenticate users via Azure ACS in this scenario).
Andrew Connell has a great article about it here