User authentication.

Hi All,

I am wondering when user is authenticated in SharePoint, Is it authenticated directly with AD or User profile service does play any role?

As per my understanding, after synchronization with AD, User Profile Service bring all the user profile in profile DB. When user is authenticated in a site collection, SharePoint bring user in to User Information List that is at site collection level.  when entry is created in user information list, does user credential also store?

if we don't have user profile service configured, can't we authenticated in SharePoint?

Regards Amit