What is a firewall?

A firewall is a piece of hardware or software that places a barrier between your network and the Internet. It prevents outsiders from accessing your network and systems and delving into your confidential information or performing malicious acts. It can further protect your systems by restricting the surfing activities of those on your network so that you aren't exposed to harmful code that they may pick up when visiting non-trusted sites.

A firewall examines each packet of data sent to your computer or network and decides, based on pre-determined parameters, whether or not to let it through. It also blocks attempts by unknown programs that may find their way onto your system from communicating with the Internet.
What doesn't a firewall do?

Firewalls don't protect your systems against viruses. For this you need anti-virus software that scans your computer on a regular basis and your emails as they arrive. These days you can get anti-virus software that integrates with firewall software so that you can monitor malicious code and intrusion attempts from the same place.

You should also keep in mind that firewalls aren't 100% effective. A good firewall will protect you against 99% of attacks, but there is always the possibility of one slipping through. There are determined hackers out there!

You need to make sure that you take other precautions to protect your systems, such as keeping to a regular back-up plan. Also remember that a firewall is only as good as its latest update, and its well worth your while making sure these are current.
What types of firewall are available?