ASP.net impersonation and delegation........

Delegation is more powerful form of impersonation,Impersonation enables ASP.NET to execute code and access resources in the context of an authenticated and authorized user, but only on the server where ASP.NET is running. To access resources located on another computer on behalf of an impersonated user requires authentication delegation. You can think of delegation as a more powerful form of impersonation, as it enables impersonation across a network.

Further details can be found @ http://msdn.microsoft.com/en-us/library/aa291350(VS.71).aspx