Facter gathers basic facts about nodes (systems) such as hardware details, network settings, OS type and version, IP addresses, MAC addresses, SSH keys and more. These facts are then made available in your Puppet manifests as variables.
Facter is extensible, and it's easy to include your own custom facts or site-specific facts. You can use the facts provided by Facter to inform conditional expressions in Puppet.
More about Facter
Get more information about Facter, including the core facts shipped with Facter and how to create your own custom facts.