iptables is the Linux firewall subsystem for the 2.4, 2.5 and 2.6 kernels.
Shorewall is a front-end software which uses iptables.
So whether you use Shorewall or Mandrake's firewall GUI, you are actually using iptables.
To learn more about iptables, go to the netfilter/iptables website.
http://www.netfilter.org/