Passport authentication can be used when ever u are using a single username password combination to authenticate in to a group of website.
The simplest example that i can give is that of a google gmail account. with a single emailid password combination u are able to access youtube,gmail,google+,blogger and most of the google web appplications.
Better you have a look at :
http://msdn.microsoft.com/en-us/library/f8e50t0f(v=vs.85).aspx[
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ASP.NET authentication and authorization[
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