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From their website:
FAI is an automated installation tool to install or deploy Debian GNU/Linux and other distributions on a bunch of different hosts or a Cluster. It's more flexible than other tools like kickstart for Red Hat, autoyast and alice for SuSE or Jumpstart for SUN Solaris. FAI can also be used for configuration management of a running system.

You can take one or more virgin PCs, turn on the power and after a few minutes Linux is installed, configured and running on all your machines, without any interaction necessary. FAI it's a scalable method for installing and updating all your computers unattended with little effort involved. It's a centralized management system for your Linux deployment.