By default, ASP.NET executes in the security context of a restricted user account on the local machine. Sometimes you need to access network resources such as a file on a shared drive, which requires additional permissions. One way to overcome this restriction is to use impersonation. With impersonation, ASP.NET can execute the request using the identity of the client who is making the request, or ASP.NET can impersonate a specific account you specify in web.config.

Implementing Impersonation in an ASP.NET Application

Choosing Resource Access Identities - Identity Impersonation In ASP.NET

MSDN: ASP.NET Impersonation

HOW TO: Secure an ASP.NET Application by Using Windows Security

On being someone else