A firewall is a network security device that blocks traffic based on a set of rules, policies, or protocols, while an IDS (Intrusion Detection System) is a network security device that detects and alerts on network threats, such as malware, intrusion attempts, or policy violations. Firewalls are designed to prevent unauthorized access to or from a private network, while IDSs are designed to detect and respond to network threats that bypass firewalls or other security measures.