A firewall is a program or hardware that prevents unauthorized access to your computer or to and from a network. The firewall acts as a barrier that only allows secure information to access your network. Any packet of information that raises a flag as being harmful based on specific criteria, it will not be allowed access.

Firewalls can monitor incoming and outgoing information to the computer network through different methods:

• Packet Filtering- Small chunks of information are analyzed against a set of filters. Information that makes it through the filters continues through a requesting systems and the others are discarded.

• Proxy Service- The firewall receives and sends information through a requesting system.

• Inspection- Certain key parts of the information is compared to a database of trusted information. If the comparison yields a match then the information is allowed through as well as vice versa.

Using one or more of these methods, a firewall can effectively protect your network from viruses or spam. For more information about Firewall's -Firewall definition from Webopedia.