It's a GPL'd perlscript that generates an iptables-restoreable iptables config. The config is located in /etc/fw/, using three files: rules.conf, global.conf and interfaces.conf. This config is read by fw-rulegen.pl. It then outputs the result on STDOUT, it just suffices to pipe it through iptables-restore and presto.
I have an init.d alike script for it to start/stop the firewall. You can find it here. Beware though, I use a homedeveloped init.d-alike system, so you will need to modify the script to make it work.