Iptables is the user-space standard for firewalling in 2.4.x linux kernels. The kernel-space standard for 2.4.x linux kernels (though ipchains and ipfwadm are still available as kernel modules) is netfilter.
The two work hand-in-hand to create a more extensive firewalling package for the linux operating system. One of the major new features allows the kernel to pass packets to the user-space for parsing and re-injection into the TCP/IP protocol stream. Iptables can be found at http://www.netfilter.org.