Answer Description
A Firewall is used to restrict network access. In it's simplest form it will block TCP and UDP ports based on number or protocol, but many modern firewalls include complex algorithms to block unusual traffic, detect possible network attacks and monitor or block traffic based on many other factors.
Wikipedia
In computing, a firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules A firewall typically establishes a barrier between a trusted internal network and untrusted external network, such as the InternetFirewalls are often categorized as either network firewalls or host-based firewalls Network firewalls filter traffic between two or more networks and run on network hardware Host-based firewalls run on host computers and control network traffic in and out of those machines
Firewall_(computing) - Wikipedia, the free encyclopedia