Authentication overview for
SharePoint
I found a very good guide from MS that explains how
users can authenticate SharePoint. It clearly explains authentication
mechanisms of SharePoint.
SharePoint 2013:
SharePoint 2010:
I hope the above information will help you to resolve the issue, in case of any queries/questions regarding the above mentioned information then please let me know. I would be more than happy to help you as well as resolves your issues, Thank you.