Need clarification on windows authentication

The main benefit of using Windows authentication is that it can be coupled with IIS authentication so that you don't have to write any custom code.
This is found in http://msdn.microsoft.com/en-us/library/ms998358.aspx
Can you tell me, if we use forms authentication, it cannot be coupled with IIS authentication? What exactly does the above line mean?
I think forms authntication can also be used with active directory username password, am I wrong?
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