SharePoint : Authentication
Authentication is the process where a computer system determines the identity of a user or other system.
First you are Authenticated to determine who you are by checking your credentials - then you are Authorized to determine what you can do
Its done by the user supplying a username and password that can be verified (Credentials).
Often this verification involves a 3rd party to 'vouch' for the user. For example an Active Directory server can vouch that a user is who they say they are so the system doesn't ever actually get to see their password or credentials.
SharePoint typically uses Windows Integrated Authentication (where your identiy is determined from the username/password you used to log in to Windows so you don't have to retype them) but can use Basic, Forms or Claims based authentication.
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