If you started talking about firewalls back in 1996, most home users would not know what you where talking about. With the sudden wide spread availability of trojans, especially with trojans like netbus and back orifice, home users had to rapidly change the way they viewed computer security.
These days most people have heard of firewalls but a lot of people do not actually know what they do. Some people also assume that the firewall that comes with their operating system is adequate to protect them from trojans, hackers and viruses. They could not be further from the truth.
So What Exactly is a Firewall? Think of a firewall as a security guard standing at a gate. The security guard checks peoples identification and looks on his list of allowed visitors. If you do not have the correct credentials and or not on the list then you are blocked from entering.
A firewall works in the same way. A user can make rules as to what connections, programs and processes are allowed to run and or connect to the internet. This provides the home user with absolute security and a good firewall can actually stop malware infections from even occurring.