A personal firewall is a software application, which filters traffic entering or leaving your computer.
When you're connected to the Internet, information is flowing to and from your PC in a near-constant stream through your PC's "ports". There are thousands of "ports" and they are like little gates that let information in and out.
Some of the information moving through the ports is obvious to you. For example, when you:
· Send and receive emails;
Access a website;
Fill in and submit a form on a website;
However, a lot of information flows to and from your PC without you knowing about it. This can include:
Your copy of Windows talking to the Microsoft website to check for security updates;
Software applications talking to their own companies' websites to check for new versions;
Anti-virus software checking for the latest virus updates.
Most of this invisible information flow is good. However, some of it poses a grave threat to your PC and your privacy.
Your email isn't the only way bad things can get into your computer. They can come in through your PC's open ports without you even knowing about them. The "Blaster" worm of August 2003 was the most widespread example of this. In case you don't remember, that was the one that caused your PC to shut down about a minute after you'd started it up!
If you leave your PC's ports open and don't monitor them, you're at extreme risk of the following nasties: