Final answer:
A firewall is a network security device that filters and controls the incoming and outgoing network traffic based on an applied rule set. It's a combination of hardware and software designed to protect networks from unauthorized access and cyber threats.
Step-by-step explanation:
A firewall is a combination of hardware and software that filters or prevents specific information from moving between the outside world and the inside world. Unlike an isolated system, which does not allow any transfer of energy or matter, a firewall selectively blocks certain kinds of data while allowing others to pass through. Firewalls are a critical component of network security, providing a barrier against cyber threats by controlling incoming and outgoing network traffic based on an applied rule set.