SharePoint Refresher: Authentication & AuthorisationPosted: 30/09/2009
SharePoint does not perform any authentication. The authentication process is taken care of by Internet Information Services (IIS). This includes:
SharePoint also supports authentication such as custom form-based authentications and authentication from an external system or delegated authentication.
Authorisation is a process that is performed and handled by SharePoint. This is where SharePoint provides security for sites, lists, libraries and items. It is important to note, that the authorisation process assumes that a SharePoint user has already been authenticated.
Further Reading: Authorisation and Authentication