Form Authentication Authenticate Request

Hi


I am very much new to form authentication.

I have to transform one existing web application using form authentication (write now application supports session based authentication).
Most of the time i have seen authenticate_request in case of form authentication.
What is that event & what we do in that event.
Also give me some URL so that i can learn about form authentication & i can implement that.
 
Thanks & Regards,
Abdul Javed Khan
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