So, what is a firewall?
For most home users, a firewall will be a piece of additional software running on the computer, which enables you to choose what comes into your computer (inbound protection) and in some cases, what goes out (outbound protection - useful to protect against spyware and adware). Think of it as being a bit like a moat around a castle - you decide who comes in, who goes out and the firewall is the drawbridge operator, acting on your instructions. In other words, it helps keep the bad guys on the Internet from accessing your computer (provided you've correctly identified who the bad guys are).

Note: If you have more than one computer on your home network, you will need to install a firewall on each computer, or direct all Internet traffic through one computer with a firewall on it.

Businesses are more likely to have a dedicated computer running their firewall software, a solution which costs more and is harder to configure but provides better protection. If you run a business which uses a permanent Internet connection, it would be advisable to investigate a dedicated hardware platform, rather than the products listed below (www.howstuffworks.com has more information and some good diagrams).

What a firewall isn't
A firewall is not a replacement for good computing practices. Online security is about prevention, detection and recovery - which will not all be achieved by a single product. A well-configured firewall will provide protection from people trying to access your computer via the Internet (many personal firewalls are hybrid products, in that they also offer detection), but you will still need a good up-to-date antivirus product to detect viruses that have made it onto your system from floppy disks, CDs and the Internet. And if the worst occurs, regular backups will make restoring your data possible. Add to this some common sense when downloading attachments and your risk is greatly reduced. Now if this sounds too much like hard work, think of it as the equivalent of door locks, a burglar alarm and house and contents insurance.