Drupal 6 and Varnish Cache
Varnish and Pressflow are two different things.. Pressflow uses varnish to improve performance.. More details goes below..

Pressflow is “a derivative of Drupal core providing enhanced performance, scalability, and data integrity”. Basically, some really smart guys at Four Kitchens and elsewhere back-ported a bunch of Drupal 7 performance enhancements to the Drupal 6 (and even Drupal 5!) code base.

Pressflow adds the following features to Drupal.


Support for database replication
Support for Squid and Varnish reverse proxy caching
Optimization for MySQL
Optimization for PHP 5


What:
Varnish is an HTTP accelerator and caching reverse proxy. Varnish is all about speed. It stores as much content as it can in the fastest place possible – RAM in this case – and bypasses the expensive process of making a request to Apache.

Why:
Pressflow structures the Drupal content to be more cache-friendly, but we still need something to actually cache the content.

How:
Varnish sits in front of Apache, accepts incoming connections from browsers and, if possible, fulfills the requests from its cache. If it can’t, it passes the request on to the underlying Apache/PHP stack. It then takes the response from Apache and forwards it on to the requesting browser. If the response from Apache is cacheable, Varnish stores it in RAM for fulfilling future requests.