[CentOS] iptables default configuration

Hi,

I would like to get some inputs on how to configure iptables. I have
referred to CentOS wiki http://wiki.centos.org/HowTos/Network/IPTables
and it has been very helpful. I have configured iptables with the help
of this tutorial, but I haven't understood few things about default
firewall config.

- What does 'RH-Firewall-1-INPUT' chain means? This also seems to be a
predefined chain, although not mentioned in wiki.
- The wiki page approach is to flush existing rules and then add
required rules to iptables. Is it possible to add/append required
rules without flushing existing set of rules? Not sure, but I think
this is where 'RH-Firewall-1-INPUT' chain comes into picture (user
defined rules).

Any explanation or resource link on this would be really helpful.

Thanks,
CS.