Poul-Henning Kamp starts his session with some history and connections with newspapers printing presses. That process had been optimalized by adding a mechanism (Heidelberg Printing Press) that delivers the papers to the press very fast. And that's kinda what Varnish does, but then with content. Varnish is a HTTP accelerator (but with better config, better management, much faster and a CMS focussed featured set). 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.

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.

The rest of his presentation uses sheets with images an graphs (eg statistics, numbers etc on server load before and after installing Varnish). Most important, Poal-Henning convinced me to try it out on one of my servers. Why haven't I used this before?

Note: I used some content about Varnish explanation from Greg's blog.
Note2: For another, more extensive live blog of this session, see davetech's blog.