As Internet worms and other malicious software tries to infect your computer across the network, a logical solution is to provide some sort of barrier against these nasties reaching the computer itself. A firewall does just this - it builds a wall against the outside world, and only lets certain known types of traffic through.

Traditionally, a firewall is a hardware network device or computer that sits between your network and the connection to the Internet; you can buy devices like this readily from computer shops, and they can do an excellent job if correctly configured.

More commonly, software firewalls are being built into operating systems . You can also download programs such as ZoneAlarm for free, which will add a firewall to your network connection. Software firewalls are somewhat less effective than the hardware variety, since the malicious software can still reach your PC, even if it can't get in. But they are easier to configure, and offer good protection against most of the worms and viruses that may attack you.