A Firewall is a system which limits network access between two or more networks. Normally, a Firewall is deployed between a trusted, protected private network and an untrusted public network. For example, the trusted network might be a corporate network, and the public network might be the Internet.
A Firewall might grant or revoke access based on user Authentication, source and destination network addresses, network protocol, time of day, network service or any combination of these. It might be implemented as an Application Level Firewall or a Packet Level Firewall. It might also implement a Content Filter, such as a Virus Wall.