There are several aspects to firewalls and the Linux Netfilter subsystem that could not be covered here. For more information, refer to the following resources.
The Official Red Hat Linux Reference Guide has a comprehensive chapter on iptables, including definitions for all command options.
The iptables manual page contains a brief summary of the various options, as well.
http://www.netfilter.org — The official homepage of the Netfilter/iptables project.
http://www.redhat.com/support/resources/networking/firewall.html — Red Hat Support firewall resource page.
http://www.tldp.org — The Linux Documentation Project contains several useful guides relating to firewall creation and administration.