Intrusion Detection Systems (IDS) have many approaches. This page is intended to document the different approaches and reference places where more information can be obtained.

Intrusion Detection Systems — http://en.wikipedia.org/wiki/Intrusion_detection_system
Intrusion Prevention Systems — http://en.wikipedia.org/wiki/Intrusion-prevention_system
Host-based IDS — http://en.wikipedia.org/wiki/Host-based_intrusion_detection_system
Application Protocol-based IDS — http://en.wikipedia.org/wiki/Application_Protocol-based_Intrusion_Detection_System
Network IDS — http://en.wikipedia.org/wiki/Nids