Authentication means, who can call or access the url of your website....
1) if you use Forms Authentication..
the authentication is based on credentials provided by User while accessing the page, inside the page login fields, and you validate them on server.
2) if its Windows..
You use your Network credentials (LDAP, Domain) to validate the user authenticity, the application will open a windows popup to enter the credentials..
2) passport is not widely used..
its used to autheticate in microsoft database, (hotmatil,live.com, etc)
go throug the article for more details
ASP.NET authentication and authorization[
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