Put simply, a firewall is a network traffic moderator. It uses a set of rules to determine what kind of traffic is allowed in and out of your computer or network. The term is a bit ambiguous, because there are many types of firewalls. In Chapter 10, we discussed the importance of using a firewall to act as a gateway into your network, denying and allowing network traffic on a network-wide basis between your computers and the outside world. This is what we refer to as a hardware appliance firewall.