Varnish is a web application accelerator. You install it in front of your web application and it will speed it up significantly.
Installation on Ubuntu
Varnish is distributed in the Ubuntu package repositories, but the version there might be out of date, and we generally recommend using the packages provided by varnish-cache.org. Please be advised that we only provide packages for Ubuntu's LTS releases, not all the intermediate releases. Note that the packages might still work fine on newer releases.
To use the varnish-cache.org repository, do the following
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
If you want to install the older 2.1 version, replace varnish-3.0 with varnish-2.1 in the command above.