An Intrusion Detection System (IDS) is a network security tool that monitors network traffic or host activities for signs of unauthorized or malicious activity. It detects and alerts about potential security incidents using techniques such as signature-based detection, anomaly-based detection, and heuristic analysis. IDS can be network-based (NIDS) or host-based (HIDS). NIDS monitors network traffic, while HIDS monitors activity on individual hosts. IDS generates alerts or logs for further investigation and response by security personnel.