There are three built-in tables for iptables: one for network filtering (filter), one for Network Address Translation (nat), and the last for specialized packet alterations (mangle). Firewall rules are organized into chains, ordered checklists of rules that the kernel works through looking for matches. The filter table has three built-in chains: INPUT, OUTPUT, and FORWARD. The INPUT and OUTPUT chains handle packets originating from or destined for the host system. The FORWARD chain handles packets just passing through the host system. The nat table also has three built-in chains: PREROUTING, POSTROUTING, and OUTPUT. mangle has only two chains: PREROUTING and OUTPUT.
netfilter checks packets entering the system. After applying any PREROUTING rules, it passes them to the INPUT chain, or to the FORWARD chain if the packet is just passing through. Upon leaving, the system packets are passed to the OUTPUT chain and then on to any POSTROUTING rules. Each of these chains has a default target (a policy) in case no match is found. User-defined chains can also be created and used as targets for packets but do not have default policies. If no match can be found in a user-defined chain, the packet is returned to the chain from which it was called and tested against the next rule in that chain.
iptables changes only the rules in the running kernel. When the system is powered off, all changes are lost. You can use the iptables-save command to make a script you can run with iptables-restore to restore your firewall settings. Such a script is often called at bootup. Many distributions have an iptables initialization script that uses the output from iptables-save