Firewalls seem to be the stuff of legend in the IT community.
This article will give you a good overview of what happens under the hood, and how you might use Linux in other security applications such as penetration-testing and intrusion detection. In its most straightforward definition, a firewall is a process that implements a set of policies that control the flow of information between one area of interest and another. It turns out that the concept of a firewall in the information-management realm goes back long before the advent of computers. Looking at what a firewall does outside the context of computing can demystify what's really going on. In banking, such procedural firewalls exist to ensure that investment bankers and stock traders don't get into a situation where one group is touting what the other is selling.