Intrusion Detection System (IDS)
Intrusion Detection System (IDS) is network security technology designed to detect vulnerability exploits. IDS detects but doesn't prevent - it monitors all inbound and outbound traffic on a device, searching for suspicious patterns of behavior that may indicate signs of a security breach.
IDS then reports its results to an administrator, but cannot actively prevent a detected exploit from taking over the system. The IDS is the 'alarm bell' for an intrusion - a firewall limits access between intrusions to prevent attacks, and the IDS sounds the alarm if it suspects an intrusion has been made.
IDS will analyze information it has gathered and compare it to large databases of attack signatures. It's more likely to be successful if it recognises a virus or attack that has already been documented, so the bigger the database, the more chance of detection. It can also detect anomalies - and will use honeypots to identify abnormal patterns of behavior.