What does Impersonation mean in .NET

Hi All,

  Can anyone explain me what is Impersonation.  I have went through some of the articles in google . But did'nt get axactly what is that ... when to use that...etc

I got the article wer its written like this ..

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.

I think u got it clearly but may b I m a tubelight .. not gettin it ..

"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"...

what these lines exactly mean...

If possible ...plz explain..

Tnx and Regards
s_raj0205