A firewall is a system designed to prevent unauthorized access to or from a private network. You can implement a firewall in either hardware or software form, or a combination of both. Firewalls prevent unauthorized Internet users from accessing private networks connected to the internet, especially intranets. All messages entering or leaving the intranet (the local network to which you are connected) must pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.
In protecting private information, a firewall is considered a first line of defence; it cannot, however, be considered the only such line. Firewalls are generally designed to protect network traffic and connections, and therefore do not attempt to authenticate individual users when determining who can access a particular computer or network.