One of the first lines of security defense that companies (and individuals) should perform is to enable a firewall. Today, most routers include a firewall, most personal-computer operating systems provide a firewall, and their are a variety of network (hardware and software) firewalls available. For this discussion:

Define the term firewall
Discuss the goal(s) of a firewall
Describe different types of firewalls (router, server, personal computer, network) and when users should use each
List several commonly available (open source) firewalls
Describe best practices for using a firewall