Vagrant is simply a wrapper around Virtualbox headless that allows for provisioning virtual machines with support for puppet, chef-solo, chef, and bash. This allows you to automate the deployment and sandboxing of development sites. Additional base box images can be found at vagrantbox.es.

He walks you through the full process of setting up a first deployment - getting Vagrant installed, updating the config for a squeeze64.box image, configuring a virtualhost on the Apache web server and setting up a "preinstall" script to configure things like MySQL and install a long list of packages (via apt-get).