A personal firewall will have a variety of levels of protection that depends on whether your computer is connected to the Internet or to a network. A computer on a home network will often have shared files with other computers on the network and will also access the same resources, such as printers. If this is the case there will be more access than if the computers are connecting to the Internet.
Connecting to the Internet is risky and needs to be controlled. Firewalls should be set to restrict certain data that is outgoing and incoming as well as close any ports that aren't being used. A port is a standardized number that routes Internet traffic with the use of network software.
You don't need to be a computer expert. Most firewall settings are configured according to default. If you need to modify these settings you'll usually get an alert when you're making the change.
There are two basic components to a personal firewall. The first component will look at the data when it comes into your computer to decide whether or not the data should be allowed to pass through. The second component will set up certain policies for some applications. These policies can be relaxed since they simply let certain programs have access to the Internet.
The first component is known as the "packet filter". This filter will take a look at each packet of data and will let it pass through or let it drop. These packets are composed of data that is routed on the network. There are rules for these packets, which are called polices, that let data pass or decide which data fails to meet the policy standards. Failure to meet these policies will give an alert or else just drop the packet depending on how the firewall has been configured.
The second component is known as the "application filter". This filter will decide whether certain programs are allowed to receive and send data through one port or another. A browser will need to have access to the Internet so a policy is put into place that allows the receiving and sending of data using Port 80, which is a standard number.
Just because the web browser is using Port 80, and other ports are blocked by the personal firewall, your computer still won't be secure.
An application filter is set up for certain programs and not the particular components of these programs. Every program will have more than one module, each of which can be infected. In a Windows environment these are known as DDLs (dynamic link libraries).
In most cases your antivirus software will be able to determine if a module is infected but any new viruses, that are not yet part of the antivirus database, may slip by undetected. Your computer system will be vulnerable if your personal firewall allows this to happen.
A firewall won't give you complete protection against viruses and hackers. A firewall is only effective for keeping some hackers out of your system and you should supplement your system with antivirus software. You should also practice safe file sharing procedures.
There is one main rule to follow with firewalls: If you don't know what is trying to accessing your computer consider it a risk.
Paul Wilcox writes about firewall software and other related topics for the Internet Security Squad website. Sign up for my free newsletter and receive my special report "How To Lock Your Computer Down - Internet Security in 2006" at http://www.internetsecuritysquad.com/freereport
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