From gbhackers.com
An intrusion detection system (IDS) is a type of security software designed to automatically alert administrators when someone or something is trying to compromise information system through malicious activities such as DDOS Attacks or security policy violations.
An IDS works by monitoring system activity through examining vulnerabilities in the system, the integrity of files and analyzing patterns based on already known attacks. It also automatically monitors the Internet to search for any of the latest threats which could result in a future attack.