From Paul Gil,
Your Guide to Internet for Beginners.
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Definition: A "firewall" is an over-arching term to describe a specialized defense system for a computer network. The term comes from construction, where specialized fire-prevention systems involve fire-resistant walls being placed strategically in buildings and cars to slow the spread of a fire. In the case of computers, the term describes hardware or software that slows the invasion of a computer system by blocking viruses and hackers.

A computer firewall itself can take hundreds of different forms. It can be a specialized software program, or a specialized physical hardware device, or often a combination of both. Its ultimate job is to block unauthorized and unwanted traffic from getting into a computer system.

Having a firewall at home is smart. You may choose to employ a software firewall like "Zone Alarm". You may also choose to install a hardware firewall "router", or use a combination of both hardware and software.

Examples of the software-only firewall: Zone Alarm, Sygate, Kerio.
Examples of a hardware firewall: Linksys, D-Link, Netgear.
Note: makers of some popular antivirus programs also offer software firewall as one security suite.
Example: AVG Anti-Virus plus Firewall Edition.

Also Known As: "sacrificial lamb server", "sniper", "watchdog", "sentry"