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 Red Hat Enterprise 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.
A list of common services and their port numbers can be found in Appendix C Common Ports and in /etc/services.
http://www.netfilter.org/ — The official homepage of the Netfilter/IPTables project.
http://www.tldp.org/ — The Linux Documentation Project contains several useful guides relating to firewall creation and administration.
http://www.iana.org/assignments/port-numbers — The official list of registered and common service ports as assigned by the Internet Assigned Numbers Authority.
Linux Firewalls, by Robert Ziegler; New Riders Press. — contains a wealth of information on building firewalls using both 2.2 kernel IPChains as well as Netfilter and IPTables. Additional security topics such as remote access issues and Intrusion Detection Systems are also covered.