When varnish is first installed it does not do much. It is a proxy, passing a web client's requests to a web server and passing the replies back. Adding a varnish configuration file tells varnish to store some of the replies in a cache and pass them back when the same questions are asked again. The server does not know anything about these repeat requests.
Varnish makes no difference to drupal version 6, the content management application. Drupal 6 has to be instructed to play nice. I can change Drupal myself or use Pressflow, a hacked copy of Drupal with the niceness built in. The Fourkitchens people supply Pressflow.