A firewall is a network security device or software that monitors and controls incoming and outgoing network traffic based on predefined security rules. It acts as a barrier between a trusted internal network and untrusted external networks, such as the internet. Firewalls help protect networks from unauthorized access, cyberattacks, and malicious traffic by filtering data packets and blocking potentially harmful connections. They are essential in preventing unauthorized users from accessing private networks and are a key component in securing IT infrastructure. Firewalls can be hardware-based, software-based, or a combination of both.