Everthing that asp.net does is through the windows account 
that is defined in machine .config which is aspnet.
Now if you want your application alone to run in a 
different account, you can change this identity to be 
changed temporarily so that the asp.net will run in another 
account(provided it is a windows account)

impersonation is the process of assigning a user account to
an unknown user.By default, the anonymous access account is
named IUSER_machinename. This account can be use to control
anonymous users' access to resources on the server.
This mechanism allows a server process to run using the
security credentials of the client. When the server is
impersonating the client, any operations performed by the
server are performed using the client's credentials.
Impersonation does not allow the server to access remote
resources on behalf of the client.

you can enable the impersonation by adding below entry in 
your application config file.

<identity impersonate="true"
          userName="domain\user" 
          password="password" />

Impersonation is nothing but identity of a client.