A firewall acts as a barrier between a network and the external world. It uses predefined rules to allow or block traffic based on criteria like source IP, destination IP, and ports. Firewalls are typically proactive and aim to prevent unauthorized access.
An Intrusion Detection System (IDS), on the other hand, is a monitoring tool that analyzes network or system traffic for suspicious activity or known attack patterns. IDS is reactive and raises alerts when it detects potential intrusions or anomalies but doesn't block traffic.