A firewall guards your computer and programs against unwanted access by hackers and malicious use. It can isolate potentially dangerous files attached to incoming emails, the source of most unwanted accesses. Options can be set to alert you to attacks or to your programs that try to access the internet. A firewall guards the ports (or doors) through which internet traffic travels.Firewalls also keep account of the programs that you allow acces to the internet. Your browser and email programs are clearly two that you do want to access the internet but others may try to gain access. You decide which programs to give internet access.
Anyone who uses a computer to send and receive email or to browse the internet should seriously consider installing a firewall. Hackers can seize your computer's internet address and use it to access passwords and credit-card numbers. Do you want this to happen?
For a free firewall download ZoneAlarm and install it yourself.