A firewall is a set of related programs, located at a network gateway server, that protects the resources of a private network from users from other networks. (The term also implies the security policy that is used with the programs.) An enterprise with an intranet that allows its workers access to the wider Internet installs a firewall to prevent outsiders from accessing its own private data resources and for controlling what outside resources its own users have access to.
Firewall software needs to be installed on a computer for it to work. The computer can either be a server to offer firewall protection to an entire network, or a workstation where only that computer will benifit from the firewall's protection.