The firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the other which exists to permit traffic. Technically, it is a system or group of systems that enforces an access control policy between two or more networks.

The Internet, like any other society, is plagued with the kind of high-tech mischieves who enjoy playing with other people's computers/networks, peeping into their PCs/Servers, or wipping their mailboxes off, or hacking their important data/code, or just sitting in their network creating havoc. Moreover, with more people shifting to DSL/CableNet, these always-on connections make networks more vulnerable to attack. Some people try to get real work done over the Internet, and others have sensitive or proprietary data they must protect. Usually, a firewall's purpose is to keep the hackers out of your network while still letting you get your job done. While, the job of the Firewall is to Protect & Guard your network, you concentrate on your business.