Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 - 1000x, depending on your architecture.
Varnish is distributed in the Ubuntu package repositories, but the version there might be out of date, and generally recommend using the packages provided by varnish-cache.org.
To use the varnish-cache.org repository, do the following
Open a Terminal and paste the following commands one by one.
curl http://repo.varnish-cache.org/debian/GPG-key.txt | sudo apt-key add -
echo "deb http://repo.varnish-cache.org/ubuntu/ precise varnish-3.0" | sudo tee -a /etc/apt/sources.list
sudo apt-get update
sudo apt-get install varnish
Here is a small diagram that simply shows how Varnish works.
