Ipchains is an older firewalling ruleset for Linux. It is a stable and effective rule making system and is still in wide use. Many performance enhancing patches exist and active development still continues. Many serious security distributions have only recently (1 year ago) changed to iptables but still offer both.
Ipchains operates by chaining rules together in order to create a comprehensive ruleset. Ipchains usually initialises the ruleset from the system V style init scripts in the /etc/init.d directory or from specialist scripts containing the initialisations and rules. It is not good practice to mix both scripts and init.d methods.
Tutorials can be found at the TLDP website.
Users of kernels from 2.4 on are encouraged to use iptables to take advantage of the "stateful" matching now offered.