Varnish is an open source ???web accelerator??? which you can use to speed up your website.
It can cache certain static elements, such as images or javascript but you can also use it for other purposes such as Load balancing or some additional security, in general most of the people want to try it and test their website to see if it’s really so amazing (IMO yes, but test it yourself).
The traditional guides will tell you to move your webserver to another port, perhaps 81,8080 or just bind to localhost, configure Varnish to listen to port 80 and use the web server as backend, the server where Varnish will forward requests not found in his cache.
This is the ???normal??? configuration and it works fine, but sometimes you just want to make a quick Test or perhaps you are using a Control Panel, such as Cpanel, Kloxo or ISPConfig and in my experience change the standard listening ports of Apache is not a decision to be taken lightly with these tools.
So in a VPS (with Kloxo) I’ve used a different approach: iptables.