there r thrr type of authentication mechanism in asp.net:

Form Authentication
Window Authentication
Passport Authentication

Authentication
Authentication refers to the method used by the server to 
verify the clients’ identity. This feature provides methods 
to authenticate clients via a set of standardized and 
reusable methods that require little or no modification. 
The methods available to developers are:
None – This method does nothing.
Windows Authentication – Attempts to verify users by 
validating supplied credentials using authentication 
methods used by the Windows operating system. This includes 
NTLM (NT LanMan) and Kerberos (for systems running versions 
more current than Windows NT 4.0). Windows authentication 
is used by default if no method is explicitly selected.
IIS Authentication – Uses authentication methods provided 
by the IIS Web server.
Passport Authentication – Verifies users through the use of 
the Microsoft Passport authentication server.
Forms Authentication – Allows for application developers to 
provide a form for authenticating users in a standardized 
way. User accounts can be made specific to the application 
and stored in the web.config file.

there are four types of Authentication in ASP.NET
Windows Auth.
Forms Based Auth.
Passport Auth.
Certificate Auth.