Squeakypedia - Firewall
A firewall is designed to protect your computer from harm by acting as an impassable wall.
However it is not the surefire solution everyone thinks it is.
A realy dedicated intruder could simply flood the firewall.
Appart from the obvious fire hazzard these walls constitute, there are also issues of overheating the computer.
Another concern in our era, is the enviromental impact firewalls has, contributing to the greenhouse effect.