What is a firewall?
A firewall is a barrier between your computer and the rest of the internet, there to make sure you are as safe as you can be from hackers, worms, viruses and all the other nasty people and programs out there.
In real life, if you look at the roof line of a row of terraced houses you will often see them, sticking up a few bricks above the tiles between each house.
A firewall on a computer does the same job, and if you have a broadband connection to the internet then you need one for your own protection.
What a firewall does
At its simplest, a firewall stops anything you don't know about passing between your computer and the internet.
So if you're using the world wide web only web-related data, like requests from your browser and pages from a server, should be moving back and forth.
And if you're e-mailing then only outgoing and incoming messages should be allowed.
That means that a hacker sitting in Chicago can't just link to your computer and look at your files or install a program they have written that will steal your credit card details.
And it means that a nasty little worm program on someone else's computer won't be able to connect to your PC and quietly copy itself.
Hardware firewalls
Hardware firewalls are separate boxes that you connect to your computer to monitor the data coming into your computer.
Hardware firewalls normally work faster than their software equivalents and often come together with other boxes you buy to connect when you want to connect more than one computer to the internet.
They are often more reliable as well but remember - hardware firewalls only look at data coming into your computer.
Software firewalls
Software firewalls are generally more tricky to setup but are more flexible and look for suspicious data going from your computer (as well as into it).
This could be useful if a virus has entered your computer via an e-mail or disk and is trying to use your internet connection to send out information you don't want it to.
If your computer is permanently connected to the internet via broadband you may want to consider using both.
How software firewalls work
When your computer is connected to the internet it can send data to and receive data from other internet-connected computers.
This data might be an e-mail message, a web page, a music file or any other sort of net content. A firewall program runs on your computer and monitors all of the data passing between you and the network.
It does this by sitting between the programs you use, like your web browser or e-mail client, and the network connection you set up when you connect to the net.
It keeps a list of 'rules' saying what can and cannot pass between you and the net. You decide what the rules are.
When you first install your firewall it will block everything. Each time you use a new program, like a web browser or chat client, the firewall will ask you if you want this program to be able to connect to the internet.
If you say 'yes' it will set up a rule to let this application go ahead, now and in future.
Getting a personal firewall
Some internet service providers, like BT and AOL, provide a free copy of a firewall program to their broadband customers.
If your ISP does this then it is sensible to use it, as you can then get technical support directly from them if anything doesn't work properly.
You can buy firewall programs from computer stores. Firewalls are sometime sold in a bundle along with anti-virus software and filtering programs to protect your children when they are online.
You can also get firewall software like ZoneAlarm by going to the supplier website and downloading it. Some of the programs are free, or you can pay for a more powerful version.
You can also go to dmoz to look at the various firewall software to download.
Keep up-to-date
Even a firewall can have holes in it, and there are regular updates to all personal firewall programs.
These will fix bugs, correct problems and sometimes add extra protection.
Whatever firewall program you use, you should keep it up-to-date.
Since firewalls are used to protect your computer when it is on the internet, the best way to keep informed of new versions and patches is to subscribe to the automatic alert service.
You will be asked if you want to do this when you first install your firewall.
Not 100% safe
If your firewall is set to block all data coming in or going out then you won't be able to use any internet programs, so you always have to open it up a little bit.
And that little bit can be enough to let in a virus or a worm, or let a hacker attack you. A virus could come attached to an e-mail. A worm might pretend to be a friendly program that you trust, like a chat server.
And a hacker might have discovered a new security hole that your firewall doesn't block yet.
So you should not rely completely on your firewall. Keep your anti-virus software up-to-date, keep backups of your data in case the worst happens and you lose it, and keep an eye out for unusual programs or activity on your computer.
A properly installed firewall, like a good burglar alarm, doesn't keep you completely safe, but it can help you sleep more peacefully at night.