1.Window
2. Forms
3. Passport

Different Type of Authentications are

Anonymous Authentication ( IUSR_COMPUTERNAME) 
Windows Integrated Authentication ( It uses NTLM or 
Kerberose, AuthType in IIS metabase)
Basic Authentication – ( Clear Text )
Digest Authentication
Advance Digest Authentication
Windows Form based authentication (using database at the 
blackened)
IIS 6 includes. Passport Authentication.
I believe Radius Authentication is one which can be 
included..! Authentication with Internet Authentication 
service IAS.

How do you retrive UserID in case of Windows Authentication?

HTTP header has the property Auth_Type = The authentication 
method that the server uses to validate users ( NTLM or 
Kerberos if Windows Intergrated Authenticaiton is used)

AUTH_USER = Returns the raw authenticated user name

Create a ASP page on webserver and drop the following lines 
to see the auth user..! 
:)

<%
for each x in Request.ServerVariables
  response.write(x & "<br />")
next
 %>

Windows integrated authentication
Anonymous authentication

We can get  the  userid  from user identity