Intrusion Detection Systems come in many forms but they all share the basic purpose of detecting and alerting based on behavior. Commonly this detection is signature based detection, similar to anti-virus solutions, though adaptive techniques are also available.
The two most common types are Network and Host based systems. NIDS are intended to be standalone devices that monitor network traffic destined for other hosts. HIDS run as applications on the end-system and often monitor both network traffic as well as system activity.