Which of the following best describes a 'firewall' in networking?

A firewall is fundamentally a security measure designed to monitor and control the flow of incoming and outgoing network traffic based on predetermined security rules. Its primary function is to create a barrier between a trusted internal network and untrusted external networks, such as the internet. By filtering traffic, a firewall helps to prevent unauthorized access, protecting sensitive data and systems from potential threats.

In essence, firewalls can be hardware-based, software-based, or a combination of both. They analyze data packets and determine whether to allow or block them, thus maintaining a secure network environment. This capability is crucial for organizations seeking to insulate their network from cyber threats.

The other options do not accurately capture the primary function of a firewall. For instance, while data backup systems are essential for data recovery, they do not provide security. Methods for optimizing network speed focus on performance rather than security and threat management. Additionally, computer virus protection is usually achieved through antivirus software, which is distinct from the role of a firewall in a network's security infrastructure.