A firewall is a network security device that acts as a barrier between an internal network and external networks, such as the internet. Its main purpose is to monitor and control incoming and outgoing network traffic based on predetermined security rules. Firewalls can be implemented as hardware appliances, software applications, or a combination of both. They examine network packets and determine whether to allow or block them based on factors like source and destination IP addresses, ports, protocols, and application-specific rules. Firewalls provide an essential layer of defense against unauthorized access, network attacks, and the spread of malicious code.