This package is largely meant to be a library for collecting facts about your system. These facts are mostly strings (i.e., not numbers), and are things like the output of uname, public ssh and cfengine keys, the number of processors, etc.
See bin/facter for an example of the interface.
Run the facter binary on the command for a full list of facts supported on your host.
See the Adding Facts wiki page for details of how to add your own custom facts to Facter.
See http://reductivelabs.com/projects/facter/ for more details.