So, what are Intrusion Detection Systems?

Intrusion Detection System (IDS) are becoming a very important part of any strategy for enterprise
security. But what are Intrusion Detection systems? CERIAS, The Center for Education and Research in Information Assurance and Security, defines it this way:

“The purpose of an intrusion detection system (or IDS) is to detect unauthorized access or misuse of a computer system. Intrusion detection systems are kind of like burglar alarms for computers. They sound alarms and sometimes even take corrective action when an intruder or abuser is detected. Many different intrusion detection systems have been developed but the detection schemes generally fall into one of two categories, anomaly detection or misuse detection. Anomaly detectors look for behavior that deviates from normal system use. Misuse detectors look for behavior that matches a known attack scenario. A great deal of time and effort has been invested in intrusion detection, and this list provides links to many sites that discuss some of these efforts”(http://www.cerias.purdue.edu/about/history/coast_resources/intrusion_detection/)

There is a sub-category of intrusion detection systems...